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/**
 * 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;
    }
}