Leetcode 142. Linked List Cycle II

Given a linked list, return the node where the cycle begins. If there is no cycle, return null.

Note: Do not modify the linked list.

Follow up: Can you solve it without using extra space?

s思路： 1. 如何用快慢指针大法检测cycle，在Leetcode 141. Linked List Cycle以及说清楚了。不说了，直接来！

//方法1：快慢指针移动大法！ class Solution { public: ListNode *detectCycle(ListNode *head) { // if(!head) return NULL; ListNode* fast=head->next,*slow=head; while(fast&&fast!=slow){ fast=fast->next?fast->next->next:NULL; slow=slow->next; } if(fast==NULL) return NULL; fast=head; slow=slow->next;//这里有一个bug：把fast放在头部，那么slow需要往下移动一位，才开始同步移动！ while(fast!=slow){ fast=fast->next; slow=slow->next; } return fast; } };