Linked List Cycle

Problem

Given the head of a linked list, return true if there is a cycle in it.

Constraints

• 0 ≤ length ≤ 10⁴
• −10⁵ ≤ val ≤ 10⁵

Approach — Two Pointers

This is a Two Pointers problem. The idea: walk two indices through the data together (or toward each other), turning an O(n²) pair search into one linear pass. Work through the reference code below line by line, then re-derive it yourself in the editor — that's how the pattern sticks.

Complexity: O(n) time, O(1) extra space.

Solution code

Python

class Solution:
    def hasCycle(self, head):
        slow = fast = head
        while fast and fast.next:
            slow = slow.next
            fast = fast.next.next
            if slow is fast:
                return True
        return False

Java

class Solution {
    public boolean hasCycle(ListNode head) {
        ListNode slow = head;
        ListNode fast = head;
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
            if (slow == fast) {
                return true;
            }
        }
        return false;
    }
}

Practice it

Reading a solution isn't the same as being able to write it under pressure. Open this problem in the in-browser editor, hide the solution, and solve it from scratch — your code runs against real test cases instantly.