leetcode-Linked List Cycle
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Given a linked list, determine if it has a cycle in it.
Follow up:
Can you solve it without using extra space?
/** * 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)return false; ListNode* node=head; ListNode* tmp=NULL; while(node->next!=NULL) { if(head==(node->next))return true; tmp=node->next; (node->next)=head;//每次把扫到的节点的next指针指向头指针。然后判断如果有节点指向头指针即存在环 node=tmp; } return false; }};
注: 一个不破换原始数据结构的方法是设两个前进速度不同的指针,例如p以一次两步(一次一步)的速度,q以一次一步(两次一步)的速度,如果没有环,指针肯定会到null,如果有环,快的指针先到达环不断绕圈,肯定存在某一时刻两个指针相遇
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