Leetcode题解141-Linked List Cycle（C++）

题目：

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

Follow up:
Can you solve it without using extra space?

思路：设立快慢两个指针，快指针一次移动两步，慢指针一次移动一步。若存在环，那么慢指针会赶上快指针。

可以想象成操场上有两个人在跑步，一个快一个慢，那么经过几圈之后，两个人肯定会相遇。

代码：

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    //141. Linked List Cycle
    //快慢两个指针，若存在环，那么慢指针会赶上快指针
    bool hasCycle(ListNode *head) {
        if(head == NULL) return false;
        ListNode* runner = head;
        ListNode* walker = head;
        while(runner->next!=NULL && runner->next->next!=NULL){
            walker = walker->next;
            runner = runner->next->next;
            if(walker == runner){
                return true;
            }
        }
        return false;
    }
};