Linked List Cycle -- leetcode

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

Follow up:

Can you solve it without using extra space?
基本思路：
快慢两指针。

一个指针每次移动一步，另一个指针每次移动两步。如存在环，必有相遇的时候。

/**
 * 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 *one = head;
        ListNode *two = head;
        while (two && two->next) {
            one = one->next;
            two = two->next;
            two = two->next;
            
            if (one == two)
                return true;
        }
        return false;
    }
};