/**
* 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) {
// len == 0 || len == 1
if (head == null || head.next == null) {
return false;
}
ListNode slow = head;
ListNode fast = head;
// if the fast pointer or the next of fast pointer
// reaches to null, end this loop
while (fast != null && fast.next != null) {
slow = slow.next;
fast = fast.next.next;
if (slow == fast) {
return true;
}
}
return false;
}
}