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

Follow up:
Can you solve it without using extra space?

思路

两个指针。一个每次走一步，另一个每次走两步，如果有环，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) {
        if(head == NULL)
            return false;
        ListNode *pre = head;
        ListNode *cur = head;
        while(pre != NULL && pre->next != NULL)
        {
            pre = pre->next->next;
            cur = cur->next;
            if(pre == cur)
                return true;
        }
        return false;
    }
};

做题是两个细节：

1. 上来判断head是否为空

2. 循环的条件是(pre != NULL && pre->next != NULL)