[leetcode] 141.Linked List Cycle

題目：
Given a linked list, determine if it has a cycle in it.
題意：
判斷一個鏈表是否存在環。
思路：
使用快慢兩個指針，一個一次走一步，一個一次走兩步。
代碼如下：

/**
 * 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* first = head, *second = head->next;
        while(first != second && second != NULL && second->next != NULL) {
            first  = first->next;
            second = second->next->next;
        }
        return (first == second);
    }
};