LeetCode 142. Linked List Cycle II

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) {        ListNode *fast = head;        ListNode *slow = head;        while ((fast != NULL) && (fast->next != NULL)) {            fast = fast->next->next;            slow = slow->next;            if (fast == slow) {                break;            }        }        if ((fast == NULL) || (fast->next == NULL)) {            return NULL;        }        fast = head;        while (fast != slow) {            slow = slow->next;            fast = fast->next;        }        return fast;    }};