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