141. Linked List Cycle

题目:

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

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

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

2/18/2017, Java

public class Solution {
    public boolean hasCycle(ListNode head) {
        if (head == null || head.next == null) return false;
        ListNode slow = head;
        ListNode fast = head.next;

        while(fast != null && fast.next != null) {
            if (slow == fast) return true;
            slow = slow.next;
            fast = fast.next.next;
        }
        return false;
    }
}