Leetcode Linked List Cycle

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

使用两个指针，一前一后，(p,q)，p与q相差一个位置，之后p移动一步，q相对应移动两步，如果存在环，那么q和p一定会相遇

/**
* 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) {
ListNode *p,*q;
if(head==NULL)
{
return false;
}
p = head;
q = head->next;
while(q!=NULL && q->next!=NULL)
{
if(p==q)
{
return true;
}
p = p->next;
q = q->next;
q = q->next;
}
return false;
}
};