链表中环的入口结点

给定一个链表，若其中包含环，则输出环的入口节点。

若其中不包含环，则输出null。

?

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

/**  * 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) {         auto a = head, b = head;                   while (b) {             a = a->next;             b = b->next;             if (b) b = b->next;             else return NULL;                           if (a == b) {                 a = head;                 while (a != b) {                     a = a->next;                     b = b->next;                 }                 return a;             }         }                   return NULL;     } };