LeetCode/kt/letter-tile-possibilities.kt

class Solution {
    private fun factorial(n: Int): Int =
        when {
            n <= 1 -> 1
            else -> (2..n).fold(1) { result, num -> result * num }
        }

    private fun permutations(seq: String): Int {
        val freqs = IntArray(26) { 0 }
        for (c in seq.toCharArray()) {
            freqs[c - 'A']++
        }

        return freqs.filter {
            it > 1
        }.fold(factorial(seq.length)) { total, count ->
            total / factorial(count)
        }
    }

    private fun generate(
        tiles: String,
        seen: MutableSet<String>,
        current: String,
        i: Int,
    ): Int =
        when {
            i >= tiles.length && seen.add(current) -> permutations(current)
            i >= tiles.length -> 0
            else -> generate(tiles, seen, current, i + 1) + generate(tiles, seen, current + tiles[i], i + 1)
        }

    fun numTilePossibilities(tiles: String): Int {
        val seen = mutableSetOf<String>()

        val letters = tiles.toCharArray()
        letters.sort()
        val sortedTiles = String(letters)

        return generate(sortedTiles, seen, "", 0) - 1
    }
}