Linked List Circle @leetcode

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

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

用最常用的办法，快慢节点各一个，一个走两步，一个走一步．如果快的那个先遇到NULL，证明无环，如果快慢节点相遇，证明有环．关键是要在循环里判断是否到尾端．

	/**
	* 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;
	if(head->next == NULL) return false;
	ListNode *p_1, *p_2;
	p_1 = head;
	p_2 = head->next;
	while(p_1!=NULL && p_2!= NULL){
	if(p_1 == p_2) return true;
	p_1 = p_1->next;
	p_2 = p_2->next;
	if(p_2 == NULL)
	return false;
	else
	p_2 = p_2->next;
	}
	return false;
	}
	};

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