141. Linked List Cycle (Easy)

https://leetcode.com/problems/linked-list-cycle/

Given a linked list, determine if it has a cycle in it.

To represent a cycle in the given linked list, we use an integer pos which represents the position (0-indexed) in the linked list where tail connects to. If pos is -1, then there is no cycle in the linked list.

Input: head = [3,2,0,-4], pos = 1
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the second node.

Input: head = [1,2], pos = 0
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the first node.

Input: head = [1], pos = -1
Output: false
Explanation: There is no cycle in the linked list.

Follow up:

Can you solve it using O(1) (i.e. constant) memory?

Solutions

1.

class Solution {
    public boolean hasCycle(ListNode head) {
        while (head != null && head.next != null) {

            // Merge the followup element until the ring remains only one element
            // which is the entry of the ring.

            head.next = head.next.next;
            if (head == head.next) {
                return true;
            }

            head = head.next;
        }

        return false;
    }
}

2.

class Solution {
    // use a set to keep all the visited elements
    Set<ListNode> visited = new HashSet<>();

    public boolean hasCycle(ListNode head) {
        ListNode node = head;
        while (node != null) {
            if (visited.contains(node)) {
                return true;
            }

            visited.add(node);
            node = node.next;
        }

        return false;
    }
}

Incorrect Solutions

References