kt: add «2375. Construct Smallest Number From DI String»

URL:	https://leetcode.com/problems/construct-smallest-number-from-di-string/
Signed-off-by: Matej Focko <me@mfocko.xyz>

kt/construct-smallest-number-from-di-string.kt

@@ -0,0 +1,28 @@
+class Solution {
+    private fun buildSequence(
+        pattern: CharArray,
+        result: StringBuilder,
+        index: Int,
+        count: Int,
+    ): Int {
+        var nextCount = count
+        when {
+            index >= pattern.size -> {}
+            pattern[index] == 'I' -> buildSequence(pattern, result, index + 1, index + 1)
+            else -> {
+                nextCount = buildSequence(pattern, result, index + 1, count)
+            }
+        }
+
+        result.append(nextCount + 1)
+        return nextCount + 1
+    }
+
+    fun smallestNumber(pattern: String): String =
+        StringBuilder().let { result ->
+            buildSequence(pattern.toCharArray(), result, 0, 0)
+
+            result.reverse()
+            result.toString()
+        }
+}