kt: add «1415. The k-th Lexicographical String of All Happy Strings of Length n»

URL:	https://leetcode.com/problems/the-k-th-lexicographical-string-of-all-happy-strings-of-length-n/
Signed-off-by: Matej Focko <me@mfocko.xyz>

kt/the-k-th-lexicographical-string-of-all-happy-strings-of-length-n.kt

@@ -0,0 +1,72 @@
+class Solution {
+    companion object {
+        private val NEXT_SMALLEST: Map<Char, Char> =
+            mapOf(
+                'a' to 'b',
+                'b' to 'a',
+                'c' to 'a',
+            )
+        private val NEXT_GREATEST: Map<Char, Char> =
+            mapOf(
+                'a' to 'c',
+                'b' to 'c',
+                'c' to 'b',
+            )
+    }
+
+    private fun variations(n: Int): Int = 1.shl(n - 1)
+
+    private fun maxHappyStrings(n: Int): Int = 3 * variations(n)
+
+    fun getHappyString(
+        n: Int,
+        k: Int,
+    ): String {
+        var k = k
+
+        // Find how many strings can be constructed
+        val total = maxHappyStrings(n)
+        if (k > total) {
+            return ""
+        }
+
+        // Initialize the resulting string
+        val result = StringBuilder(n)
+        (1..n).forEach {
+            result.append('a')
+        }
+
+        val startA = 1
+        val startB = startA + variations(n)
+        val startC = startB + variations(n)
+
+        when {
+            k < startB -> {
+                result.setCharAt(0, 'a')
+                k -= startA
+            }
+            k < startC -> {
+                result.setCharAt(0, 'b')
+                k -= startB
+            }
+            else -> {
+                result.setCharAt(0, 'c')
+                k -= startC
+            }
+        }
+
+        (1..n - 1).forEach { index ->
+            val mid: Int = 1.shl(n - index - 1)
+
+            when {
+                k < mid -> result.setCharAt(index, NEXT_SMALLEST[result[index - 1]]!!)
+                else -> {
+                    result.setCharAt(index, NEXT_GREATEST[result[index - 1]]!!)
+                    k -= mid
+                }
+            }
+        }
+
+        return result.toString()
+    }
+}