［Leetcode］141. Linked List Cycle

题目：

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

Follow up:

Can you solve it without using extra space?

让head不断走一步，如果cur不为null，走两步，这样如果存在cycle，cur会追上head。

/**
 * 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* cur = head;
        
        while (cur && cur->next != head){
            cur = cur->next;
            if (cur)
                cur = cur->next;
            else
                break;
            head = head->next;
        }
        if (!cur) return false;
        return true;
    }
};