[leetcode]Linked List Cycle

新博文地址：[leetcode]Linked List Cycle

http://oj.leetcode.com/problems/linked-list-cycle/

 

这道题是很经典的面试题，在腾讯阿里笔试，网易面试中都遇到过原题。。。。双指针，不多说了

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

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

 

    public boolean hasCycle(ListNode head) {
		if (head == null) {
			return false;
		}
		ListNode first = head;
		ListNode second = head;
		while (true) {
			try {
				first = first.next;
				second = second.next.next;
			} catch (NullPointerException e) {
				return false;
			}
			if (first == second) {
				return true;
			}
		}
    }

 

两个指针，一个每次走一步，另一个每次走两步，如果存在环的话，肯定会遇到的，如果不存在环的话，往后走肯定会遇到空指针的。