141.leetcode Linked List Cycle(easy)[链表是否有环 快慢指针]

Given a linked list, determine if it has a cycle in it.

Follow up:

Can you solve it without using extra space?

题目的意思是判断一个链表是否有环，判断的方法是用快慢两个指针，一个指针每次走两步，一个指针每次走一步，当两个指针相遇时那么存在环。

/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
bool hasCycle(ListNode *head) {
if(head == NULL) return false;
ListNode* p = head,*q = head;
while(p != NULL&&q != NULL)
{
p = p->next;
q = q->next;
if(q != NULL)
q = q->next;
if(p == q && p != NULL)
return true;
}
return false;
}
};