141. Linked List Cycle

/**
 * 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;
    }
}