Linked List Cycle (Medium)

Description

Given a linked list, determine if it has a cycle in it.
Follow up:
Can you solve it without using extra space?

Analysis

O(1) Space.循环遍历每个节点。把每个节点的next断开指向自己。如果这种情况出现环。
则说明原来就有环存在

My Solution

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
//C++
/**
 * 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* p = head;
        while(head->next){
            p = head->next;
            if(p==head)
                return true;
            head->next = head;
            head = p;
        }
        return false;
    }
};