Linked List Cycle II - Leetcode Solution

 

Que:- Given the head of a linked list, return the node where the cycle begins. If there is no cycle, return null.

There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the next pointer. Internally, pos is used to denote the index of the node that tail's next pointer is connected to (0-indexed). It is -1 if there is no cycle. Note that pos is not passed as a parameter.

Do not modify the linked list.

Approach

Floyd Cycle algorithm is used in this Solution

Complexity

• Time complexity:- O(N-L),where N is the number of nodes in Linked List and L is the length of loop

• Space complexity:- O(1)

Code

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *detectCycle(ListNode *head) {
        // FLoyd Cycle algorithm
        // Time complexity is O(N-L), where N is the number of nodes in Linked List and L is the length of loop, and Space
        // complexity is O(1).
        ListNode *slow = head;
        ListNode *fast = head;
        bool flag = false;

        while(fast!=NULL && fast->next!=NULL && fast->next->next !=NULL)
        {
            slow = slow->next;
            fast = fast->next->next;
            if(slow==fast)
            {
                flag = true;
                break;
            }
        }
        if(!flag)
            return NULL;
        slow = head;
        while(slow!=fast)
        {
            slow = slow->next;
            fast = fast->next;
        }
        return slow;
    }
};

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