原题：
Given a linked list, determine if it has a cycle in it.
代码如下；
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* struct ListNode *next;
* };
*/
bool hasCycle(struct ListNode *head) {
if(head==NULL)
return false;
struct ListNode* p1;
struct ListNode* p2;
p1=head;
p2=head;
while(p2->next&&p2->next->next)
{
p1=p1->next;
p2=p2->next->next;
if(p1==p2)
return true;
}
return false;
}
写两个差异指针就好咯。
