Leetcode 142. Linked List Cycle II (Medium) (cpp)

Leetcode 142. Linked List Cycle II (Medium) (cpp)

Tag: Linked List, Two Pointers

Difficulty: Medium

/*142. Linked List Cycle II (Medium)Given a linked list, return the node where the cycle begins. If there is no cycle, return null.Note: Do not modify the linked list.Follow up:Can you solve it without using extra space?*//** * 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) {        if(head == NULL || head -> next == NULL) {            return NULL;        }        ListNode *s = head, *f = head;        while (f != NULL && f -> next != NULL) {            s = s -> next;            f = f -> next -> next;            if (s == f) break;        }        if (s != f) return NULL;        while (head != s) {            head = head -> next;            s = s -> next;        }        return head;     }};