leetcode链表--5、linked-list-cycle(单链表是否有环)

题目描述

 

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

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

 

解题思路：使用快慢指针，若在遍历过程中slow==fast则有环，否则无环

/**
 * 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)//记得空链表处理
            return false;
        ListNode *pSlow = head;
        ListNode *pFast = head;
        while(pFast->next != NULL && pFast->next->next != NULL)
        {

            pSlow = pSlow->next;
            pFast = pFast->next->next;
            if(pSlow == pFast)//一定要移动完再判断
                return true;
        }
        return false;
    
    }
};