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?

题目大意：判断一个给定链表是否存在环

题目难度：Easy

/**
 * Created by gzdaijie on 16/6/17
 * 两个指针从头开始走,一个一次走一步,另一个一次走2步,如果存在环,必定会相遇
 */
public class Solution {
    public boolean hasCycle(ListNode head) {
        if (head == null || head.next == null) return false;

        ListNode faster = head.next;
        while (faster.next != null && faster.next.next != null) {
            if (head == faster) return true;
            head = head.next;
            faster = faster.next.next;
        }
        return false;
    }
}