[leetcode][list][two pointers] Linked List Cycle II

题目：

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?

/**
* 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) {
//是否有环
if(NULL == head || NULL == head->next) return NULL;
ListNode *slow = head;
ListNode *fast = head->next;
while(fast && fast->next && slow != fast){
slow = slow->next;
fast = fast->next->next;
}
if(slow != fast) return NULL;//无环
//计算环中元素个数n
int cnt = 1;
slow = slow->next;
while(slow != fast){
++cnt;
slow = slow->next;
}
//前后指针，一个指针先走n部，然后两个指针同步向前，相遇点就是入口
ListNode *front = head;
ListNode *behind = head;
for(int i = 0; i < cnt; ++i) front = front->next;
while(front != behind){
front = front->next;
behind = behind->next;
}
return front;
}
};