package leetCode_142;

/**

* @author dimdark

* @date 2017-09-09

* @time 10:59 AM

*/

public class Solution {

static class ListNode {

int val;

ListNode next;

ListNode(int val) {

this.val = val;

}

}

public ListNode detectCycle(ListNode head) {

if (head == null || head.next == null) return null;

ListNode fast = head, slow = head, entry = head;

while (fast.next != null && fast.next.next != null) { // non-cycle if not true

slow = slow.next;

fast = fast.next.next;

if (fast == slow) { // encounter

while (entry != slow) {

entry = entry.next;

slow = slow.next;

}

return entry;

}

}

return null; // non-cycle

}

}