Given a linked list, determine if it has a cycle in it.
This is Floyd's Linked List Cycle algorithm, or Tortoise and the Hare algorithm with 1 pointer moving faster than the other until they meet.
/**
* 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; // tortoise pointer
ListNode fast = head.next; // hare pointer
// race both pointers until they meet
while (fast != null && fast.next != null) {
if (slow.val == fast.val) return true;
slow = slow.next;
fast = fast.next.next;
}
// no cycle
return false;
}
}