Day 3 using Kotlin - Advent of Code 2025

 https://adventofcode.com/2025/day/3

 

var max: Char
var maxIndex: Int
var startSub: String
var trailSub: String
var startMax: String
var trailMax: String

readInput( "Day03" ).sumOf { line ->
    max = line.max()
    maxIndex = line.indexOf( max )
    startSub = line.take( maxIndex )
    trailSub = line.substring( maxIndex + 1 )
    startMax = if ( startSub.isEmpty()) max.toString() else
        startSub.max().toString() + max
    trailMax = if ( trailSub.isEmpty()) max.toString() else 
        max.toString() + trailSub.max()
    startMax.toInt().coerceAtLeast( trailMax.toInt())
}.println()

 

 

 --- Part 2 ---

 

val digitCount = 12
var indexCharPairList: MutableList<Pair<Int, Char>>

readInput( "Day03" ).sumOf { line ->
    var remainCount = digitCount

    indexCharPairList = mutableListOf()

    fun findMax( subLine: String, startIndex: Int ) {
        max = subLine.max()
        maxIndex = subLine.indexOf( max )

        indexCharPairList.add( Pair( maxIndex + startIndex, max ))

        remainCount--

        if ( remainCount > 0 ) {
            startSub = subLine.take( maxIndex )

            if ( maxIndex < subLine.lastIndex ) {
                val trailIndex = maxIndex + 1

                trailSub = subLine.substring( trailIndex )

                if ( trailSub.length <= remainCount ) {
                    indexCharPairList.addAll(
                        trailSub.mapIndexed { index, char ->
                            index + trailIndex + startIndex to char
                        } // mapIndexed
                    ) // addAll

                    remainCount -= trailSub.length

                    if ( remainCount > 0 ) {
                        findMax( startSub, startIndex )
                    } // if
                } else {
                    findMax( trailSub, trailIndex + startIndex )
                } // if - else
            } else {
                findMax( startSub, startIndex )
            } // if - else
        } // if
    } // fun findMax( String, Int )

    findMax( line, 0 )

    indexCharPairList.sortedWith { one, other ->
        one.first - other.first
    }.map { pair ->
        pair.second
    }.joinToString( "" ).toLong()
}.println()