You are given a very large integer n, represented as a string,​​​​​​ and an integer digit x. The digits in n and the digit x are in the inclusive range [1, 9], and n may represent a negative number.

OptimisedAddNumberAsMaxValue.kt

package org.kotlinlang.play

fun main() {
    val input = mapOf(
        -999 to -5999,
        999 to 9995,
        9643 to 96543,
        -132 to -1325
    )
    val added = 5
    
    input.keys.forEach {
        val result = maxValue(it.toString(), added)
        println(result)
    }
}


fun maxValue(n: String, x: Int): String {
    var result: String
    val numberLength = n.length
    val digits = n.toCharArray()

    val until: Int
    if (digits.first() != '-') {
        val items = digits.withIndex().filter { Character.digit(it.value, 10) < x }
        until = if (items.isNotEmpty()) { items.first().index } else { numberLength }

        result = digits.copyOfRange(0, until).joinToString("")
    } else {
        val items = digits.withIndex().filter { t -> Character.digit(t.value, 10) > x  }
        until = if (items.isNotEmpty()) { items.first().index } else { numberLength }

        result = digits.copyOfRange(0, until).joinToString("")
    }

    result += ('0' + x)

    result += digits.copyOfRange(until, numberLength).joinToString("")

    return result
}

Playground:
https://pl.kotl.in/XbWmjtSHW

Leet code:
https://leetcode.com/problems/maximum-value-after-insertion/submissions/