LeetCode--Linked List Cycle

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) {         /*        1. 空链表不成环        2. 一个节点自环        3. 一条链表完整成环        */        if(head==NULL)return false;        if(head->next==NULL)return false;        if(head->next==head)return true;        ListNode* y=head->next;        ListNode* x=head->next->next;        while(x!=NULL && y!=NULL)        {            x=x->next;            if(x==NULL)break;            x=x->next;            y=y->next;            if(x==y)break;        }        return x==y;    }    };

标准做法是使用两个指针，一个每次往前走2步，一个每次往前走1步，如果两个指针相遇，即说明链表有环，时间复杂度为O(N)，空间复杂度为O(1)。