My Java solution with o(1) time complexity and o(1) space complexity:

static SinglyLinkedListNode insertNodeAtHead(SinglyLinkedListNode llist, int data) {
        SinglyLinkedListNode newHead = new SinglyLinkedListNode(data);
        newHead.next = llist;
        return newHead;
    }

python 3

def insertNodeAtHead(llist, data):
    new_node = SinglyLinkedListNode(node_data=data)
    if llist is None:
        return new_node
    else:
        new_node.next = llist
        return new_node

Here is my c++ solution you watch vide explanation here : https://youtu.be/COnfpdhOlN8

SinglyLinkedListNode* insertNodeAtHead(SinglyLinkedListNode* llist, int data) {
   SinglyLinkedListNode* new_node = new SinglyLinkedListNode(data);
   new_node -> next = llist;
   return new_node;
}

Java Solution

static SinglyLinkedListNode insertNodeAtHead(SinglyLinkedListNode llist, int data) { SinglyLinkedListNode node = new SinglyLinkedListNode(data); if (llist != null){ node.next = llist; } llist = node; return llist; }

Just a warning to anyone writing this in C# c sharp: there is an error in the main function that makes solving this problem using C# impossible.

PrintSinglyLinkedList(llist->head, "\n", textWriter);

should be

PrintSinglyLinkedList(llist.head, "\n", textWriter);