LinkedListCycle.java

/*
 Author:     King, higuige@gmail.com
 Date:       Oct 5, 2014
 Problem:    Linked List Cycle
 Difficulty: Easy
 Source:     http://oj.leetcode.com/problems/linked-list-cycle/
 Notes:
 Given a linked list, determine if it has a cycle in it.
 Follow up:
 Can you solve it without using extra space?

 Solution: two pointers.
*/

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public boolean hasCycle(ListNode head) {
        if (head == null || head.next == null) return false;
        ListNode slow = head, fast = head;
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
            if (slow == fast) break;
        }
        if (fast == null || fast.next == null) return false;
        return true;
    }
}