[Kotlin] Algorithm - Combination

private const val alphabets = "ABC"
private val results = mutableListOf<Char>() // Altered Alphabet


fun main() {
    combination(2)
}

/**
 * Combination
 * @param count: Number to be extracted
 * @param index: The index to begin the search. Numbers already extracted do not need to be checked again because they do not take into account the order.
 * @param depth: Number of Extracted
 */
fun combination(count: Int = 0, index: Int = 0, depth: Int = 0) {
    if (depth == count) {
        println("[${results.joinToString(", ")}]") // a complete combination
        return
    }

    for (i in index until alphabets.length) {
        results.add(alphabets[i])
        combination(count, i + 1, depth + 1)
        results.removeAt(results.lastIndex)
    }
}

fun main() {
    combination(arrayOf("A", "B", "C"), 2).forEach {
        println(it)
    }
}

fun <T> combination(elements: Array<T>, r: Int): List<List<T>> {
    val n = elements.size
    val results = mutableListOf<List<T>>() // the number of all cases

    val result = elements.sliceArray(0 until r)

    fun recursionCombination(depth: Int = 0, index: Int = 0) {
        if (depth == r) {
            results.add(result.toList())
            return
        }

        for (i in index until n) {
            result[depth] = elements[i]
            recursionCombination(depth + 1, i + 1)
        }
    }

    recursionCombination()
    return results
}

[A, B]
[A, C]
[B, C]