题目链接：https://leetcode.com/problems/linked-list-cycle-ii/description/

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.

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