# Cycle Detection

# Cycle Detection

clacrone + 50 comments The old "tortoise and the hare" problem. Essentially, the "fast" pointer moves 2x the speed as the "slow" pointer. If there is a cycle, they will meet.

`int HasCycle(Node head) { if (head == null){ return 0; } Node slow = head; Node fast = head; while (fast != null && fast.next != null){ slow = slow.next; fast = fast.next.next; if (slow == fast){ return 1; } } return 0; }`

Tofurkey + 1 comment Great solution and explanation!

byronrpv_rckr + 2 comments can't get the explanation, do not catch the analogy, there is no such tale in my country i guess, ja ja plz can someone explain me the code a little bit?? thanks men

corey_t_ferguson + 2 comments The tale isn't important.

There is a "slow" pointer which moves one node at a time. There is a "fast" pointer which moves twice as fast, two nodes at a time.

A visualization as slow and fast pointers move through linked list with 10 nodes:

1: |sf--------| 2: |-s-f------| 3: |--s--f----| 4: |---s---f--| 5: |----s----f|

At this point one of two things are true: 1) the linked list does not loop (checked with

`fast != null && fast.next != null`

) or 2) it does loop. Let's continue visualization assuming it does loop:6: |-f----s---| 7: |---f---s--| 8: |-----f--s-| 9: |-------f-s| 10: s == f

If the linked list is not looped, the fast pointer finishes the race at O(n/2) time; we can remove the constant and call it O(n). If the linked list does loop, the slow pointer moves through the whole linked list and eventually equals the faster pointer at O(n) time.

RA1511003010655 + 3 comments Sorry this might seem silly but as soon as fast reaches the last node it will exit right? I mean this is a singly linked list so for fast to reach back to slow, how is that possible? (if there is a cycle that is)

ghost_009 + 0 comments If there is no loop in the linked list then

**fast.next**will become**null**after reaching the end of linked list and the while loop will exit without ever marking the flag as 1. And if there is a loop it will catch up with**slow**as it is moving twice as fast and as soon as**fast**will be equal to**slow**the flag will be marked as 1 and finally the while loop will exit.nicolasfont + 0 comments If there's a cycle, the fast pointer will never reach an end since there is no node with no next node, and it will eventually meet the slow pointer.

MrAndrew7of9 + 0 comments I think your confusion is like mine in at first assuming that the fast node is skipping a node while it is going "twice as fast". In reality it is not as

`fast.next.next`

is actually referencing the next node and then the next after that. If either`.next`

is in a cycle/loop, fast will go through that cycle/loop and slow will also edventually go through it too. They will eventually converge without us needing to know how long the linked list is. Since the problem statement restricts the max is 1000, you can simply keep a count and exit once the limit is reached. Which is computationally more efficient. The slow and fast counter is the best way if we don't know the size restrictions.Count tracker example:

`static boolean hasCycle(SinglyLinkedListNode head) { // since the list size limit is 1000, you can change this based on your max limitations in specific use cases int maxSize = 1000; int count = 0; while(head!=null) { head = head.next; count++; //if cycle exists count will increase forever in this while loop if(count>maxSize){ //to stop our loop from running forever have a max contition check return true; } } return false; }`

kumarmukul123451 + 0 comments what if the loop starts from f and points to any pointer before s in fig 1. can u show its correctness??

amritgg977 + 0 comments its called

**Floyd’s Cycle detection algorithm**. you can search for it for better explanation.You can watch videos also at*https://www.youtube.com/results?searchquery=Floyd%E2%80%99s+Cycle+detection+algorithm*

Mr_BrainTurner + 0 comments nice one

jens_vanderhaeg1 + 1 comment What would you say the time complexity is? O(N)?

Abushawish + 2 comments Yes, O(N) because you have to go through every element regardless of factor.

hammeramr + 11 comments would this be O(1) // because we know the max size is 100 and O(100) = O(k) = O(1)

boolean hasCycle(Node head) { if(head == null || head.next == null) return false; Node n = head; // list max size = 100 so after 100 steps if n != null return true for(int i = 0; i < 100; i++) { if(n == null) { return false; } else { n = n.next; } } return true; }

ekuznets + 0 comments [deleted]khandelwalnites1 + 0 comments [deleted]Madhav_Bhatt + 0 comments No, it will still be O(n), that's the definition of big-O notation. Simply think like below. k values(i/p size) loop rotation 1 1 100 100 n n

Hence order is n. We should not take input size constant while calculating time complexity.

usmanajmal + 1 comment Yes, this should be O(1). She didn't take it constant. The given question has provided a constant max size of 100 for the linked list.

ashu_msr19 + 0 comments n is a variable used in big O notation for unknown test cases. if n=1, O(1). if n=100, O(100). if n=?, O(n).

unwrittenwater4 + 0 comments [deleted]ashu_msr19 + 1 comment [deleted]maitrey2112 + 0 comments Wow!. Man your grammar is incredibly stupid and incorrect , it's good you can at least code.

dharmikp572 + 0 comments I did similar thing in java XD

rahuldev83 + 0 comments LOL

bhupankarg + 2 comments Compare addresses between current node own address and current node pointing address. If the pointing address is less than current node own adress and the pointing address is not null then definitely it's pointing to a traversed node and return true otherwise return false. Because in data structure element's addresses are taken in accending order.

160102012cse + 1 comment no bro it is not working

bhupankarg + 1 comment No, it will work. Please refer....!

bool has_cycle(SinglyLinkedListNode* head) { SinglyLinkedListNode* temp = head; while (true) { if(temp->next < temp) return (temp->next == nullptr) ? false : true; temp = temp->next; } }

My submission: https://www.hackerrank.com/challenges/detect-whether-a-linked-list-contains-a-cycle/submissions/code/104878239

160102012cse + 0 comments i havent thought about the final i.e whose next is null thanks bro

marszym + 0 comments Actually it is not deterministic for the allocator to always allocate memory at higher addresses. I belive that two nodes which are chronologically in order can reside in addresses which are of the reverse relation (the older node can have a lower address).

This kind of solution is heavily reliant on the storage provided for the linked list and the allocator at hand. The task does not specify this kind of information so it is

**incorrect**to assume that this is a**valid**solution.

abhinavkushagra + 1 comment Not necessarily. It mayn't find the cycle in first go. So, we can't say it's of O(n).

brainiak + 0 comments First learn what's an O(n)

nike3801 + 0 comments Nice explanation.

askarovdaulet + 1 comment Great except the first three lines of the function are redundant. You will still fall through to return 0, because in case head is null, fast = null and you skip the while loop altogether.

tamas_csiszer2 + 0 comments [deleted]

hekuang + 2 comments But what if the last pointer points back to the first one?

askarovdaulet + 0 comments [deleted]askarovdaulet + 1 comment Will still work. Imagine there are only two elements [head,tail] and tail.next points to head. Then on the second iteration of while loop, slow==fast==head and you return 1. You can show that in general you will detect the loop of tail back to head on the n-th iteration of the while loop where n is the total number of elements.

hekuang + 0 comments Right! Thanks for the explanation.

ethan_sec + 5 comments In this case, I feel this is an overkill. There's a constraint that the list is <=100 nodes. So, why traverse the list several times by definition until they meet.

I solved it (in Python) with a counter:

def has_cycle(head): count = 0 while head and count<101: count += 1 head = head.next return count>100

rouzbeh_asghari1 + 5 comments Thanks for your solution. How about this solution. I tried to make it more general. What if we don't know about 100?

visited = [] def has_cycle(head): if head: visited.append(head.data) if head.next: if head.next.data in visited: return True else: has_cycle(head.next) else: return False else: return False

shubhampachori6 + 2 comments What if two nodes in a list have the same data in two nodes but they are not in a cycle?

ishabandi + 5 comments Then store the node address? My solution works on the test cases. Have a look?

def has_cycle(head): if head is None: return False visited = [] node = head flag = 99 while node is not None: if node not in visited: visited.append(node) else: flag = 1 return True node = node.next if flag==99: return False

Worst case here will be O(n^2) right?

adityaloshali1 + 0 comments [deleted]adityaloshali1 + 1 comment this works perfectly

def has_cycle(head): x = [] while head: if head in x: return True else: x.append(head) head=head.next return False

ishabandi + 2 comments Yes! yours is cleaner. too many redundant lines in mine. What about the complexity? Will it work under huge lists?

adityaloshali1 + 0 comments Yes it will work for huge lists too . Either this algorithm , or use the Rabbit and Hare algorithm (fast and slow pointer) . that can reduce the time for travelling the list . but either way it's a brute force algorithm . you'll have to travel till the end of list to identify the presence of a cycle

thebopshoobop + 0 comments You should use a set rather than a list. If you check the wiki, you'll see that

x in s

operations on lists are O(n) on lists but average O(1) on sets, while adding to both is O(1). Therefore, as written this algorithm is O(n^2), but would be O(n) with a set.

jcflorezr + 0 comments [deleted]ojhasaurabh2099 + 0 comments [deleted]abhi_kind + 0 comments how n^2 this work only in o(n)

jayprakashchawl1 + 0 comments try storing addresses instead of data

gregigusa + 0 comments [deleted]vshantam + 1 comment Yes ,Your Solution is much more general and understandable but you used too much conditionals here is my approch

def has_cycle(head): temp=head;visited=[] while temp.next!=None: if temp.data in visited: return True else: visited.append(temp.data);temp=temp.next return False

josef_klotzner + 0 comments only to look up "data" is not sufficient. "data" can be a lot of times in a list. you need to store and look up for whole object or use a tuple holding (data, next). Together with its "next" it is unique and if you find this again you got a loop --> True

maitrey2112 + 0 comments If you don't care about the list after the program(like here) , you can reduce worst case complexity to O(N) and in other case O(k), where k is no. of nodes you have visited , it'll be better than O(100) and O(n^2).

def has_cycle(head): x = head y = head while x: if (x.next=="visited"): return True else: y = x.next x.next="visited" x=y return False

henryjw + 0 comments `visited = set()`

It's probably a good idea to use sets instead of arrays for lookups since the difference is

`O(n)`

vs`O(1)`

lookup time. It doesn't matter for the size in this problem, but it'll make a difference in problems with larger input sizes.

DishaParwani + 0 comments [deleted]NorthernLight + 0 comments The 'slow' pointer

*never*traverses the list more than once if you solve using the 'tortoise and hare' method. The worst case (entire list is traversed once) occurs when the cycle starts at the header node.You see, since the number of nodes between the 'slow' and the 'fast' pointer decreases by 1 every iteration (in a cycle), once the 'slow' pointer reaches the first node of the cycle, it has to traverse only as many more nodes as the number between the two pointers before they meet(Remember, the number is counted starting from the 'fast' node, and in the direction it's travelling (clockwise/anticlockwise)).

dougbeck + 0 comments I had a similar solution, except recursive and js. Not sure if there is a bug in this code or the solution checker:

var count = 0; var max = 100; function hasCycle(head) { if (head === null) { return false; } if (count > max) { return true; } count++; return hasCycle(head.next); }

edit: I guess there is a problem with state. Removing the recurrsion and encapsulating the counter passes

`¯\_(ツ)_/¯`

HHWWHH + 0 comments Another way in python:

def has_cycle(head): cur=head while cur!=None: pointer=cur while pointer!=None: if pointer.next!=cur: pointer=pointer.next else: return 1 cur=cur.next return 0

positivedeist_07 + 0 comments I did exactly as per ur logic but I get compilation errors. The output is not the expected one. It shows segmentation fault :(

and, should we not make use of the fact that the list is restricted to size 100?

pls help me out!!

anirudh04 + 1 comment If I use "while(fast.next!=null && fast!=null)" It's not working.Can anyone explain this?

rajp4690 + 0 comments you should check "fast != null" first because if fast is null then as per your condition it will be like "null.next != null" and you can't check null.next that is why its not working.

alpha52omega41 + 0 comments Also known as Flyod Cycle finding algorithm..

ishabandi + 0 comments I hope my understanding is correct.

You are just checking if any of the 'next' are same by using to iterables?

benraw11 + 0 comments [deleted]mjhere + 0 comments Good solution. Thank you:)

chanchanman + 1 comment Yes! The Floyd Algorithm! :D

elaines + 0 comments Why it's related to this algo? :) I checked floyd, but didn't get any clue of their similarity. could you explain a bit?

MoulikGupta + 0 comments The old "tortoise and the hare" problem in python.

MoulikGupta + 1 comment The old "tortoise and the hare" problem in python.

def has_cycle(head): cycle=False slow=head fast=head while head and head.next: if slow==fast: cycle=True break slow=slow.next fast=fast.next.next return cycle

premmkrishna + 0 comments while head and head.next - Wouldn't this be a infinite loop in case of a perfect LL as we are not altering it.

naitik0212 + 1 comment If there are just two nodes in the linked list with a cycle, how will it work?

yash324 + 0 comments slow will increment by 1 and fast will increment by 2, they will be the same every alternate loop.

abhinavmane113 + 0 comments Impressive solution. Btw Add null check for fast pointer after if statement in while loop, in some cases segmentaion fault occurs.

rollins_jackson + 0 comments Is this classic CS material or something?

prempaljee111 + 0 comments bro im confused of if condition bcoz when fast is updated loop will again begins then when will if condition be checked . plz guide me

zzmmss + 0 comments Easy stroy and deep Zen. Incredible!

ShubhamK27 + 1 comment See this one has O(1) complexity

bool has_cycle(Node* head) { if(head==NULL||head->next==NULL) return head; int index=0; Node* temp= head; while(temp && index<101){ index++; temp=temp->next; } if(index==101) return 1; return 0; }

abhinavkushagra + 1 comment (-

*_*_-) Seriously? You're kidding us? You lose your O(1) the time you used while loop.ShubhamK27 + 1 comment Boy! The order is independent of the size. It does not matters whether you use N=2/3/6...100. Every time its gonna take the same time bro! A consant time is always of O(1). Isn't it?

abhinavkushagra + 1 comment No! Time is constant, yes. But size is'nt. The code that you've created by no means, will give you constant time for different sizes. I mean, your time complexity depends on the size.

ShubhamK27 + 1 comment Okay! If there will be a loop, it will continue till the index reaches 101 from 0 but if no loop is there it will depend on the size. So its of order O(1) for looped one and O(n) for non looped. Now am I correct?

abhinavkushagra + 0 comments Not exactly! :\

kharcheavinash11 + 0 comments [deleted]andy17 + 0 comments Hi, why it isn't working with the code

`while(slow!=NULL && slow.next!=NULL) In both sense I think it should work,pls help me out.`

dimahwang1 + 0 comments Thank you for great solution, however why is it the case that in C++ fast hast to point to head's next as opposed to both slow and fast pointing to head in Java. Thanks.

bool has_cycle(Node* head) { // Complete this function // Do not write the main method if(head == NULL) return false; Node* slow = head; Node* fast = head->next; while(fast != NULL && fast->next != NULL) { slow = slow->next; fast = fast->next->next; if(fast == slow) return true; } return false; }

gan_mit08 + 0 comments [deleted]sarath_chandra99 + 0 comments in while loop instead of fast if i replace slow, its not working..can u tell me y? and y did u use (fast != null && fast.next != null) , just (fast!=null) is giving output

email109 + 1 comment Indeed, this solution is correct. Problem is - it doesn't pass the test. I guess their test is broken for this problem.

diegocast04 + 0 comments I have the same issue...

eternity_is_all + 0 comments Why segmentation error ?

mayank7577 + 2 comments Sir this solution is not working it is giving null pointer exception!

eternity_is_all + 0 comments `bool has_cycle(SinglyLinkedListNode* head) { SinglyLinkedListNode* p=head; SinglyLinkedListNode* q=head; if(p==NULL) return false; while(p!=NULL && p->next!=NULL) { q=q->next; p=p->next->next; if(p==q) return true; } return false; }`

eternity_is_all + 0 comments Here is the solution that works fine for me. The while condition is important here otherwise pointer error will arise

mayank7577 + 1 comment This solution is not working in Java! Compiler message 'Wrong Answer'.

engfragui + 0 comments Same for me

manish2421 + 0 comments this is not working on list of 1213512135. please provide solution

viragjain_vj + 0 comments I write the code in the same way, but its failing for the testcases and when i paste the same code written by you, still the testcase are failing. Is there any bug in the testcases? May be their expected output written wrong if your code is correct.

nihar_turumella + 0 comments hey for the while check, I used slow instead of the fast ... basically slow !=Null && slow.next!=Null and except for the first two cases, the ret failed, why?

JavistSetia + 0 comments 5/7 test cases failed using this algo!!!!!

samundarsingh5 + 0 comments [deleted]manshitomarleo + 0 comments [deleted]manshitomarleo + 1 comment bool has_cycle(SinglyLinkedListNode* head) { if(!head) return false; SinglyLinkedListNode* ptr1=head; SinglyLinkedListNode* ptr2=head;

`while(ptr1->next!=NULL||ptr1!=NULL){ ptr2=ptr2->next; ptr1=ptr1->next->next; if(ptr1==ptr2) return true; }`

return false;} this is my code. exactly similar. but it gives segmentation error. please Help!

john357 + 0 comments You have an || instead of &&

[deleted] + 1 comment while (slow != null && fast != null && fast.next != null ) should be check..

preetkewl + 1 comment `static boolean hasCycle(SinglyLinkedListNode head) { SinglyLinkedListNode fast = head, slow = head; while( fast != null && slow != null && fast.next != null){ slow = slow.next; fast = fast.next.next; if(slow == fast){ return true; } } return false; }`

preetkewl + 0 comments Still not able to clear all test case.

yanjuntong + 0 comments This is also the one recommended in "Cracking the Coding Interview" However it timed out in when I try the same approach.

piyush1751995 + 0 comments Cant we make the fast pointer run faster (like 3 jumps)??

153suman153 + 0 comments [deleted]vijayrathod8422 + 0 comments great!!

mohammednausheed + 0 comments In the while condition we should add one more condition so as to pass the rest of the testcases. It should go like this , while(fast.next!=null&&fast!=null&&fast.next.next!=null) ;)

pc06matt + 0 comments This solution is great for small list, but not really good for use-cases with cycle and super large cycle. Because catching up with slow, may have to visit all nodes in edge cycle cases. Hashing looks better solution though

nomenoparty99 + 0 comments How to compare 2 nodes, each node has 1 data and pointers point to. If this code is true so we can compare 2 node by the pointer which point to it right ?

deepamgupta + 0 comments Good Concept

tanzeelsiddiqui + 0 comments what if the second last node of the linked list is looped with the head or the second node (there can be many cases) then, it can't detect the loop.

MrAndrew7of9 + 0 comments I don't believe your first check if the head is null is necessary because you default return 0 and the while loop is the only way to return 1 which checks if fast (initially set to head) is not null.

Zolbayar + 3 comments **The predefined codes in Java 7, Javascript, Python 2, Python 3 are not working!**kapploneon + 1 comment Is the testcase 1 failing. I am facing similar problem, I think my solution in Java is correct. I tried debugging.

Zolbayar + 1 comment You should try it with C# or another language.

kapploneon + 0 comments You mean to check my logic is correct or not?

c4ar1_1lo0s5 + 0 comments Thank god I saw your comment, I tried my approach in C and it works now, I spent almost three hours thinking what was wrong with my code. Trying to debug Java with custom input is useless too.

sungmkim80 + 0 comments This is still happening with JavaScript. Ended up solving it in C#.

sgoel01 + 5 comments I took two pointers from head. Increased one pointer by 2 with every iteration and another pointer with 1. If there is loop they will somehow meet in the loop and will return 1. If any of them encounters NULL return 0 :)

sidthekidder + 0 comments Wow, that's something unique

kr_abhinav + 0 comments What you are talking about is floyd-cycle finding algo. If you found it yourself,commendable!

krishnkant_ + 3 comments i've got another way: run a counter variable (k) to count the no. of nodes we have transversed in the linked list and if any node in the linked list points NULL (while k<100) then the list surely does not contain any cycle.

i_rnb + 0 comments @krishnkant_ Sure that's a way, but that's cheat. In practical situation, you never know the maximum length of the linked list. Floyd's cycle detection algo is the most elegant solution.

hugo_ratiney + 0 comments Smart !

gowthamprakaash1 + 0 comments true logic is correct not a cheat if it contains cycle,then it will cycle cycle cycle....

nripesh21 + 0 comments elegant!

ujjwalsaxena + 0 comments Thanx!! did the same and Voilaa..

boolean hasCycle(Node head) { Node n=head; Node n1=head; boolean present=false; while(n!=null && n.next!=null) { n=n.next.next; n1=n1.next; if(n==n1) { present=true; break; } } return present; }

hshah8831 + 6 comments Can someone please explain the input 1, and the output 01101. I understand that there are 5 linked list given as input, and each input integer in output says whether the corresponding lined ist has cycle or not. I am getting output 01111. can any of the moderator give the 4th input linked list. It would be helpful in debugging.

littleninja + 3 comments Edit: The problem was (of course) in my logic. I had two references in the list, but they needed to start at different points so that they didn't fail the first check.

My check compares

*Node.data*and I suspect there may be a non-unique value in the list. All I see for input for Testcase 0 and Testcase 1 is "1". I'm using the Java language option. Frustrating.sothy_chan + 2 comments Not sure what's up with the testcase either, but the reason why it's not passing is because it says "revisiting a node." A node consists more than just the node.data. Two different nodes can have the same data. So to check node.data is the wrong logic.

littleninja + 3 comments Correct, but that's a red herring. My first attempt did a Node==Node check and this is what I returned to. I tried Node.data==Node.data as I was troubleshooting, but this isn't a good way to test (since it fails if the linked list has duplicate data values). The problem with my implementation was where I started the two node references. I initialized both at

*head*, did my checks, then incremented at different speeds. Obviously if they start at the same position and I check to see if the nodes are the same, they're going to fail. The way I fixed it was by initializing one node at*head*and the other at*head.next*. Solved.ak47gyani + 1 comment I dint understand. If you start both pointers at head and increment them at different speeds, then why ll it fail?

mohitaggarwal516 + 1 comment it will fail in the first case only. It wont go further.

krishzinx + 0 comments cant we first increament the pointers and then check??

varad2512 + 0 comments The simpler way just change the way you compare.Instead of node->data compare the node's addresses.Works!

nijwm21 + 0 comments ur same logic applied but i am getting runtime error in testcase 0.can u please tell me whats the wrong with my code ?

pnanda + 0 comments Yes you are correct. I just wasted 20 minutes trying to debug this issue. In my case i had a slow pointer and a fast pointer (running at 2x speed of slow pointer). Thanks !

VikashYadav + 5 comments You don't need to compare data as in a cyclic loop all entries may be different(explained in 2nd test case). My code iff you want to check- Node current=head; for(int i=0;i<100;i++){

`if(current==null){ return 0; } current=current.next; } return 1;`

virajh + 1 comment If the code above is your solution, it is very incorrect since the assumption is nodes will not be greater than 100.

t_egginton + 0 comments no, it says 0 <= listSize <= 100. Now, unless their variable names are chosen terribly you could assume that this means there is never more than 100 nodes in the list. therefore if you iterate through 100 times and don't hit a NULL node then you know it must be a cycle

Gorakh_Nigam + 0 comments what if cycling has been done twice??

manni_reies + 0 comments LOL, I came up with the same solution, it's a cheat solution thought lol :P

Yeetesh + 0 comments haha i came to the same solution but it felt really noob to use it :P

positivedeist_07 + 0 comments Can u pls explain what u did here corresponds to the question asked?

RaoPisay + 0 comments Well I did it in java using hashCode()

ja5hga1a + 2 comments Was this issue resolved or the solution ever found??? I am facing the same issue. Exactly the same output as yours.

sothy_chan + 1 comment My solution was to store the node in an array list, the node, not just node.data. Then check to see if the list already contains the node.

agodfrey3 + 0 comments if instead of an array list, you use a hash set, you can reduce the complexity down to O(n) because lookup in a hash set is a constant time operation instead of logarithmic or linear in your array ( depending if your array is sorted or not) .

littleninja + 0 comments Depends on how you're trying to solve, but I figured out what was wrong with my implementation. I posted my fix in another comment: https://www.hackerrank.com/challenges/detect-whether-a-linked-list-contains-a-cycle/forum/comments/63711

VikashYadav + 0 comments The output is final output after testing all sample cases but the input shown are the inputs till which your program continues be executing without giving any runtime error.And most probably your program is giving null pointer exception in first test case itself therefore it is not showing further inputs to be tested.

Bonniesaur + 0 comments [deleted]singh_mona + 2 comments I also got the same output. And I am not able to undrstand the input. My code:

bool has_cycle(Node* head) { if(head==NULL) return false; if(head->next=head) return true; Node *slow=head; Node *fast=head->next; while(slow&&slow->next) { if(slow==fast) return true; slow=slow->next; fast=fast->next->next; } return false; // Complete this function // Do not write the main method }

How to olve this issue?

edward_ribeiro + 0 comments hey,

`if(head->next=head)`

is not mean to be

`if(head->next==head)`

you are using

`assignment`

instead of`equality`

.Also, your while loop is evaluating

`slow`

and`slow->next`

that are pointers to memory address, they are`!= 0`

(that is, true). I guess you mean`while (slow != NULL && slow->next != NULL)`

, right? Finally,`fast`

reaches the end of the list before slow (if it does not contains cycles), so you should check for`fast`

in the while expression, don't you?Cheers!

rishindrareddy + 0 comments [deleted]

Piyushpky + 0 comments there are two cases where the loop breaks 1st one is when fast pointer reaches to NULL(which means while(fast->next!=NULL) ) and 2nd is when slow==fast. check these condition. if(fast->next==NULL) return false; else true;

Thuxg + 1 comment The codes has bug ( Python2 and Python3 ): Sorry: TabError: inconsistent use of tabs and spaces in indentation (solution.py, line 80)

I cant compile it !!!

singhgursheesh12 + 0 comments Working now

eduardoacjobs + 7 comments Problem solving this exercise the main function contains "int" declaration on javascript code making fail the execution

function main() { .... for (let testsItr = 0; testsItr < tests; testsItr++) { .... for (int i = 0; i < llistCount; i++) { if (i == index) { extra = temp; } if (i != llistCount-1) { temp = temp.next; } } .... }

Could you fix this issue or tell me where I can submit a pull request to solve it? Thanks

victorjavierss + 1 comment I also ran into the same issue with JS.

emibel + 0 comments Same issue. still failing in JS due 'int'

gphalen52 + 1 comment yep, just ran into this^ I mean, I can do it in another language, but I actually just figured it out in Python and wanted to see if the same solution converted to JS.

eduardoacjobs + 1 comment We need to be able to edit the code or they have to fix it :)

shishirsharma + 0 comments ` solution.js:99 for (int i = 0; i < llistCount; i++) { ^

SyntaxError: Unexpected identifier at createScript (vm.js:80:10) at Object.runInThisContext (vm.js:139:10) at Module._compile (module.js:616:28) at Object.Module._extensions..js (module.js:663:10) at Module.load (module.js:565:32) at tryModuleLoad (module.js:505:12) at Function.Module._load (module.js:497:3) at Function.Module.runMain (module.js:693:10) at startup (bootstrap_node.js:188:16) at bootstrap_node.js:609:3

eehoops + 0 comments Plus one to this

nick171 + 0 comments 10 months later and nobody fixed it.

shadab14meb346 + 0 comments I requested an edit also but nothing .

diego_viniegra + 0 comments I have to learn C in order to solve this... nooO!!!!

kentdang_it + 0 comments How could you get the llistCount if it is cycle?

neonbjb + 1 comment The Java pre-defined parser for this problem is broken - use another language until it works.

mukeshj89_only + 0 comments Yes , right !!

LsK101 + 0 comments **Can't complete in JavaScript due to a bug, counter variable in for-loop of code checker is declared with "int".**

emarsc + 2 comments The Node JS code is broken, the python code is broken, and I am getting incorrect results with a java function that I know works. This new site layout is not good.

rjbcc58 + 1 comment Same, I am using java HashSet to check if node was previously seen (not fast/slow pointer algorithm), but cannot get correct results.

shofi384 + 3 comments I have tried the Java HashSet as well as the slow and fast node approach; both gave me wrong answer. I looked into the main method; I think the main method is never creating a cycle but appending each node to the end of the list. That's why test for cycle fails. Though the same algo and similar functions worked in C++

anikakelhanka + 0 comments I also faced the same problem. Even used the editorial solution. That failed too.

amarbatp + 1 comment Yeah, it is exactly same to me. I've tried with 3 different approaches, no difference. Hidden code looks not creating any cycle anyway. Did anyone overcome this issue?

kapploneon + 1 comment Nope

amarbatp + 0 comments Thanks for answering, Kapploneon. Finally, I've accomplished it by writing my code on C++ instead Java. Pre-made code for Java has some problem, but it is well developed for C++.

amarbatp + 0 comments RESOLVED: After struggling with Java for a while, finally I've wrote my code on C++ and successfully submitted. It seems like hidden Java code for this problem is not developed well, so you won't have chance to accomplish it on Java.

maucaro + 0 comments Node is broken; I know what is wrong but the editor does not let me edit the existing code outside my function.

WeilanYang + 7 comments Well, I didn't know the floyd algo... I could only think of using a Hashset. I don't know if it's allowed to import java.utils, but things becomes insanely easy if you switch to python.

`def HasCycle(head): nodes = set() while (head != None): if (head in nodes): return 1 nodes.add(head) head = head.next return 0`

arunkumar_123 + 0 comments In my first approach, I was trying to use a hash table to to keep track of all the nodes visited till now. As soon as we get a node which is already present in hash table.. we can say that linked list contains a cycle. However, this approach is relately complex and requires some extra space. Fast and slow pointer method of finding loop in a linked list data structure is better.

hugo_ratiney + 0 comments I did the same, came here to see if there was a better way... was not disappointed !

Abushawish + 3 comments Did it using HashSet in Java, no problem.

boolean hasCycle(Node head) { if (head == null) return false; Node hare = head; HashSet<Node> nodesVisited = new HashSet<Node>(); while (hare != null) { if (nodesVisited.contains(hare)) return true; nodesVisited.add(hare); hare = hare.next; } return false; }

m_balajee + 0 comments [deleted]saif01md + 1 comment do the last statement always return false ?? what is the meaningof return false in the last line.I think it should always print false as it is hard coded in the last line.

festmeny + 0 comments Inside the while loop if there's a node that has been visited before, we return true as the list contains a cylce:

if (nodesVisited.contains(hare)) return true;

If hare beceomes null in one point, it means that the list has been fully visited (there's no cylce in it), so the while loop brakes. The last line runs only if the while broke, which happens only if there's no cycle in the list.

kapploneon + 0 comments Hey I applied similar solution tried with storing Node as well as with hashcode() of the node both are failing for the testcase 1. I don't know why. Compiler message: Wrong answer. output 0, expected 1.

Though I tried debbugging and none of the elements repeat.

m_balajee + 0 comments [deleted]m_balajee + 0 comments [deleted]m_balajee + 1 comment What if the node points to itself?

Abushawish + 0 comments [deleted]

DanHaggard + 0 comments [deleted]

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