Linked List Cycle

Given head, the head of a linked list, determine if the linked list has a cycle in it. There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the next pointer. Internally, pos is used to denote the index of the node that tail's next pointer is connected to. Note that pos is not passed as a parameter.

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

Source

package questions

import _utils.UseCommentAsDocumentation
import questions.common.LeetNode
import utils.shouldBe

/**
 * Given head, the head of a linked list, determine if the linked list has a cycle in it.
 * There is a cycle in a linked list if there is some node in the list that can be reached again
 * by continuously following the next pointer.
 * Internally, pos is used to denote the index of the node that tail's next pointer is connected to.
 * Note that pos is not passed as a parameter.
 *
 * Return true if there is a cycle in the linked list. Otherwise, return false.
 *
 * [Source](https://leetcode.com/problems/linked-list-cycle-kt/)
 */
@UseCommentAsDocumentation
private fun hasCycle(head: LeetNode?): Boolean {
    if (head == null) return false
    var slowPointer = head
    var fastPointer = head.next

    while (slowPointer != null || fastPointer != null) {
        if (slowPointer == fastPointer) {
            return true
        }
        slowPointer = slowPointer?.next
        fastPointer = fastPointer?.next?.next
    }
    return false
}

private fun LeetNode.getFirst(value: Int): LeetNode? {
    var current: LeetNode? = this
    while (current != null) {
        if (current.`val` == value) {
            return current
        }
        current = current.next
    }
    return null
}

fun main() {
    hasCycle(head = LeetNode.from(3, 2, 0, -4)
        .also {
            it.getFirst(-4)!!.next = it.getFirst(2) // add cycle
        }) shouldBe true

    hasCycle(head = LeetNode.from(1, 2)
        .also {
            it.getFirst(2)!!.next = it.getFirst(1)
        }) shouldBe true

    hasCycle(head = LeetNode.from(3, 2, 0, -4)) shouldBe false

}

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Updated on 2021-11-06