[leetcode 142]Linked List Cycle II

Given a linked list, return the node where the cycle begins. If there is no cycle, return null.

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) {            return NULL;        }        auto slow = head;        auto fast = head;        while (fast && fast->next) {            slow = slow->next;            fast = fast->next->next;            if (slow == fast) {                auto slow2 = head;                while (slow2 != slow) {                    slow = slow->next;                    slow2 = slow2->next;                }                return slow2;            }        }        return NULL;    }};