142 Linked List Cycle II(如果链表有环，找到入口结点)

 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     ListNode *next;
 6  *     ListNode(int x) : val(x), next(NULL) {}
 7  * };
 8  */
 9 class Solution {
10 public:
11     ListNode *detectCycle(ListNode *head) {
12         if(head==NULL)return NULL;
13         ListNode *p1,*p2;
14         p1=p2=head;
15         int n=1;
16         while(p2->next&&p2->next->next){
17             p1=p1->next;
18             p2=p2->next->next;
19             if(p1==p2){
20                 while(p1->next!=p2){
21                     p1=p1->next;
22                     ++n;
23                 }
24                 p1=p2=head;
25                 while(n--){
26                     p2=p2->next;
27                 }
28                 while(true){
29                     if(p1==p2)return p1;
30                     p1=p1->next;
31                     p2=p2->next;
32                 }
33             }
34         }
35         return NULL;
36     }
37 };