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 circlewhile(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;}
};

转载于:https://www.cnblogs.com/superzrx/p/3420817.html

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