/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *entryNodeOfLoop(ListNode *head) {
        //思路，主要还是分析其中的性质，用代码实现反而没那么难
        if(!head || !head->next) return nullptr;
        auto first = head,second = head;
        while(first && second){
            first = first->next;
            second = second->next;
            if(second)    second = second->next;
            else return nullptr;

            //相遇与c点
            if(second == first){
                first = head;
                while(first != second){
                    first = first->next;
                    second = second->next;
                }
                return first;
            }
        }
        return nullptr;
    }
};