带环链表-LintCode

给定一个链表，判断它是否有环。
思想：
设置两个指针(fast, slow)，初始值都指向头，slow每次前进一步，fast每次前进二步，如果链表存在环，则fast必定先进入环，而slow后进入环，两个指针必定相遇。

#ifndef C102_H#define C102_H#include<iostream>using namespace std;class ListNode{public:    int val;    ListNode *next;    ListNode(int val){        this->val = val;        this->next = NULL;    }};class Solution {public:    /**    * @param head: The first node of linked list.    * @return: True if it has a cycle, or false    */    bool hasCycle(ListNode *head) {        // write your code here        ListNode *fast = head, *slow = head;        while (fast&&fast->next)        {            slow = slow->next;            fast = fast->next->next;            if (slow == fast)            {                return true;                break;            }        }        return false;    }};#endif