LeetCode-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?

首先确定是否有环，并找到环上一点，通过该点得到环的长度，用两个差为环长度的指针来探测环的起点。

/**
 * 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) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        ListNode * p1=head,*p2=head;
        ListNode * pic,*ptr;
        if(head==NULL)return NULL;
        int length=1;//length of the circle
        while(true){
            p1=p1->next;
            if(p1==NULL)return NULL;
            p2=p2->next;
            if(p2==NULL)return NULL;
            p2=p2->next;
            if(p2==NULL)return NULL;
            if(p1==p2){
                pic=p1;
                ptr=p2;
                break;
            }
        }
        while(ptr->next!=pic){
            length++;
            ptr=ptr->next;
        }
        if(length==1)return pic;
        p1=head;
        for(int i=0;i<length;i++){
            p1=p1->next;
        }
        p2=head;
        while(p1!=p2){
            p1=p1->next;
            p2=p2->next;
        }
        return p1;
    }
};