There is an important part of the task that I missed: "permutation of the first N natural numbers"

Here are few test cases to further help understand the problem: (as the provided test cases do not cover K > 1)

• Test Case 1

Input:
5 2
4 3 2 5 1

Intermediate:
After first swap (of 5 and 4): 5 3 2 4 1
After second swap  (of 3 and 4): 5 4 2 3 1

Output:
5 4 2 3 1

• Test Case 2

Input:
5 2
3 4 2 5 1

Intermediate:
After first swap (of 5 and 3): 5 4 2 3 1
After second swap  (of 2 and 3): 5 4 3 2 1

Output:
5 4 3 2 1

Jave O(k) using index array

public class Solution {

public static void main(String[] args) {
    Scanner input = new Scanner(System.in);
    int n = input.nextInt();
    int k = input.nextInt();
    int[] a = new int[n];
    int[] index = new int[n + 1];
    for (int i = 0; i < n; i++) {
        a[i] = input.nextInt();
        index[a[i]] = i;
    }
    for (int i = 0; i < n && k > 0; i++) {
        if (a[i] == n - i) {
            continue;
        }
        a[index[n - i]] = a[i];
        index[a[i]] = index[n - i];
        a[i] = n - i;
        index[n - i] = i;
        k--; 
    }
    for (int i = 0; i < n; i++) {
        System.out.print(a[i] + " ");
    }
}

}

c++ it works

#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;

int main()
{
ios::sync_with_stdio(false); cin.tie(0); 
long long int n,k,flag=0;
cin>>n>>k;
long long int i,j=0,arr[n],brr[n],temp;
for(i=0;i<n;i++)
{
cin>>arr[i];

}

for(i=0;i<n;i++)
{
if(arr[i]!=n-i&&k!=0)
{
j=i+1;
while(arr[j]!=n-i)
j++;
temp=arr[i];
arr[i]=arr[j];
arr[j]=temp;
k--;

}
}

for(i=0;i<n;i++)
cout<<arr[i]<<" ";
cout<<"\n";
return 0;
}

The success rate is only 60% for this problem. Don't know why it is marked "easy". I think this should be "medium"