LeetCode -- LinkedListCycle

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) {}

 * };

 */

用两个指针，一个每次前进1步，一个每次前进2步，如果存在环，那么两个指针就终会相遇。要注意判断快慢针前进过程中是否会是NULL。

class Solution{
public:
bool hasCycle(ListNode *head){

ListNode *pointer1 = head;
ListNode *pointer2 = head;

while(1){
if(pointer1 == NULL) return false;

pointer2 = pointer2 ->next;
if(pointer2 == NULL) return false;

pointer2 = pointer2 ->next;
if(pointer2 == NULL) return false;

if(pointer2 == pointer1)return true;

pointer1 = pointer1 ->next;
}
}
}