LeetCode 141. Linked List Cycle

这个题目的只要设置一个快步走的指针和一个慢步走的指针即可，如果有环，两者一定会重逢

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

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:
    bool hasCycle(ListNode *head) {
        if (head == NULL || head->next == NULL||head->next->next==NULL)return false;
        ListNode *p = head->next,*s=head->next->next;
        while (s&&s->next)
        {
            if (p == s)return true;
            p = p->next;
            s = s->next->next;
        }
        return false;
    }
};