kt: add «3272. Find the Count of Good Integers»

URL:	https://leetcode.com/problems/find-the-count-of-good-integers/
Signed-off-by: Matej Focko <me@mfocko.xyz>

kt/find-the-count-of-good-integers.kt

@@ -0,0 +1,50 @@
+class Solution {
+    private fun precomputeFactorial(n: Int): LongArray =
+        LongArray(n + 1) { 1 }.let { fs ->
+            (1..n).forEach { i ->
+                fs[i] = fs[i - 1] * i
+            }
+
+            fs
+        }
+
+    private fun countDigits(s: String): IntArray =
+        IntArray(10).let { digit ->
+            s.toCharArray().forEach { digit[it - '0']++ }
+            digit
+        }
+
+    private fun getPalindromes(
+        n: Int,
+        k: Int,
+    ): Set<String> =
+        buildSet {
+            val base = (1..(n - 1) / 2).fold(1) { acc, _ -> acc * 10 }
+            val skip = n.and(1)
+
+            for (i in base..10 * base - 1) {
+                var s = i.toString()
+                s += StringBuilder(s).reverse().substring(skip)
+
+                val palindrome = s.toLong()
+                if (palindrome % k == 0L) {
+                    val chars = s.toCharArray()
+                    chars.sort()
+                    add(String(chars))
+                }
+            }
+        }
+
+    fun countGoodIntegers(
+        n: Int,
+        k: Int,
+    ): Long =
+        precomputeFactorial(n).let { factorial ->
+            getPalindromes(n, k).sumOf { s ->
+                val digit = countDigits(s)
+                val total = (n - digit[0]) * factorial[n - 1]
+
+                digit.fold(total) { total, count -> total / factorial[count] }
+            }
+        }
+}