141. Linked List Cycle

#linked-list #cycle-detection

Problem

Linked List Cycle - LeetCodeLeetCode

Intuition

Time Complexity

Space Complexity

Solution

/**
 * 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) {
        // n -> number of nodes in linked list
        // m -> Number of loop rotations (generally a very small number)
        // Time Complexity: O(n), traverse through the list once
        // It could be O(m * n), but the number of loop rotations could be a very small number

        // Space Complexity: O(1), auxillary space, we are not allocating any extra space

        // If the head is null, there is no cycle, so return false
        if (head == null) {
            return false;
        }

        // Initialize two pointers, walker and runner, both pointing to the head of the linked list
        ListNode walker = head;
        ListNode runner = head;

        // Move the runner pointer by two steps and the walker pointer by one step
        // If there is a cycle, the runner will eventually catch up with the walker
        while (runner != null && runner.next != null) {
            walker = walker.next;
            runner = runner.next.next;

            // If the walker and runner meet, there is a cycle, so return true
            if (walker == runner) {
                return true;
            }
        }

        // If the loop completes without finding a cycle, return false
        return false;
    }
}