LeetCode/kt/remove-nodes-from-linked-list.kt

class ListNode(var `val`: Int) {
    var next: ListNode? = null
}

fun toListNode(vararg xs: Int): ListNode? {
    if (xs.isEmpty()) {
        return null
    }

    val dummy = ListNode(0)
    var node = dummy

    for (x in xs) {
        val next = ListNode(x)
        node.next = next
        node = next
    }

    return dummy.next
}

fun linkedListEquals(
    head: ListNode?,
    xs: List<Int>,
): Boolean {
    var node = head
    for ((i, x) in xs.withIndex()) {
        if (node == null) {
            println("[DEBUG] $x is expected at index $i in the linked list, but is not present")
            return false
        } else if (node.`val` != x) {
            println("[DEBUG] $x is expected at index $i in the linked list, but ${node.`val`} is present")
            return false
        }

        node = node.next
    }

    if (node != null) {
        println("[DEBUG] Linked list is longer than expected")
    }

    return node == null
}

class Solution {
    fun removeNodes(head: ListNode?): ListNode? = removeNodesRec(head).first

    private fun removeNodesRec(head: ListNode?): Pair<ListNode?, Int> {
        if (head == null) {
            return head to Int.MIN_VALUE
        } else if (head.next == null) {
            return head to head.`val`
        }

        val (next, rightMax) = removeNodesRec(head.next)

        var node = head
        if (head.`val` < rightMax) {
            node = next
        } else {
            node.next = next
        }

        return node to maxOf(head.`val`, rightMax)
    }
}

fun main() {
    val s = Solution()

    check(linkedListEquals(s.removeNodes(toListNode(5, 2, 13, 3, 8)), listOf(13, 8)))
    check(linkedListEquals(s.removeNodes(toListNode(1, 1, 1, 1)), listOf(1, 1, 1, 1)))
}