Problem List

Linked List Cycle

May 31, 2025 • Go • linked list, hash table, two pointer • easy

Problem

Return true if there is a cycle in the linked list. Otherwise, return false.

Approach

1. Keep looping the one node fast as twice as the other until they either run into eachother or exit the linked list

Reflections

I also did the hashmap version

func hasCycle(head *ListNode) bool {
    m := make(map[*ListNode]bool)

    node := head
    for node != nil {
        if m[node] {
            return true
        }
        m[node] = true
        node = node.Next
    }

    return false
}

Go Solution

func hasCycle(head *ListNode) bool {
    slow,fast := head,head
    for fast != nil && fast.Next != nil {
        slow = slow.Next
        fast = fast.Next.Next
        if slow == fast {
            return true
        }
    }

    return false
}

LeetCode Problem Link