Detect and Remove Loop

Posted: 10 Dec, 2019
Difficulty: Moderate

PROBLEM STATEMENT

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Given a singly linked list, you have to detect the loop and remove the loop from the linked list, if present. You have to make changes in the given linked list itself and return the updated linked list.

Expected Complexity: Try doing it in O(n) time complexity and O(1) space complexity. Here, n is the number of nodes in the linked list.

Input format:

The first line of input contains two values: the number of nodes in the linked list and the value of the kth node from which the last node connects to form the loop while the second line of input contains the given linked list.
The value of k should be greater than or equal to 0 and less than equal to n. For, k equal to 0, there is no loop present in the linked list and for k equal to n, the last node is connected to itself to form the cycle.

Output Format:

The only output line contains the linked list after removing the loop if present.
Constraints:
1 <= N <= 100000.
1 <= ‘VAL’ <= 1000 .  

Time limit: 1 sec
Approach 1

We are going to have two loops outer-loop and inner-loop 

  1. Maintain a count of the number of nodes visited in outer loop.
  2. For every node of the outer-loop, start the inner loop from the head and maintain a  prev node to store the previous node of the current outer loop node.
  3. If the inner-loop visits the node next to the outer-loop node, then a cycle exist,we will simply set the prev node's next as NULL that will break the cycle and return the linked list else repeat the process for the next iteration of outer-loop.
  4. If outer-loop reaches the end of list or null, then no cycle exist, return head.
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