kt: add «1079. Letter Tile Possibilities»

URL:	https://leetcode.com/problems/letter-tile-possibilities/
Signed-off-by: Matej Focko <me@mfocko.xyz>

kt/letter-tile-possibilities.kt

@@ -0,0 +1,42 @@
+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
+    }
+}