Leetcode--Linked List Cycle

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

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

下面是错误的解法：

声明两个指针cur nex ，一个（cur）指向头指针，另一个（nex）遍历直到cur==nex 或 nex==NULL

/** * 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||head->next==NULL)            return false;        ListNode *cur=head;        ListNode *nex=head->next;        while(nex!=NULL){            if(cur==nex)                return true;                        nex=nex->next;        }        return false;    }};

Submission Result: Time Limit Exceeded

这个错误用这个图可以解释：

程序执行这种情况就是个死循环

Solution:

/** * 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||head->next==NULL)            return false;        ListNode *fast=head;        ListNode *slow=head;        while(fast!=NULL&&fast->next!=NULL)//fast->nex有意义的前提是fast不为NULL        {            slow=slow->next;            fast=fast->next->next;            if(slow==fast)                return true;        }        return false;    }};