Linked List Cycle - facebook code lab - pass test case - study code: https://siddontang.gitbooks.io/leetcode-solution/content/linked_list/linked_list_cycle.html

LinkedListCycle2.java

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     public int val;
 *     public ListNode next;
 *     ListNode(int x) { val = x; next = null; }
 * }
 */
public class Solution {
    /*
       1. if there is no cycle, return null
       2. if there is a cycle, use two runner, meet a same node; 
          and then, third runner starts from begining to meet the normal 
          runner at the beginning of cycle. 
       study the code: 
       https://siddontang.gitbooks.io/leetcode-solution/content/linked_list/linked_list_cycle.html
    */
	public ListNode detectCycle(ListNode a) {
	    if(a == null || a.next == null) {
            return null;
        }

        ListNode fast = a;
        ListNode slow = a;

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

        return null;
	}
}