1. 题目

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

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


2.解决方案


class Solution {
public:
    bool hasCycle(ListNode *head) {
        if(!head){
        return false;
    }

    ListNode* slowPoint = head;
    ListNode* fastPoint = head;

    while(slowPoint->next){
        //slow point move one step
        slowPoint = slowPoint->next;
        //fast point move two step
        if(fastPoint->next){
            fastPoint = fastPoint->next;
        }else{
            return false;
        }
        if(fastPoint->next){
            fastPoint = fastPoint->next;
        }else{
            return false;
        }
        if(slowPoint->val == fastPoint->val){
            return true;
        }
    }
    return false;
    }
};


思路：用快慢指针来走，如果有环的话，一定会遇到一起。