kt: add «3163. String Compression III»

URL:	https://leetcode.com/problems/string-compression-iii/
Signed-off-by: Matej Focko <me@mfocko.xyz>

kt/string-compression-iii.kt

@@ -0,0 +1,32 @@
+class Solution {
+    private data class State(val compressed: MutableList<Pair<Int, Char>>, val count: Int, val character: Char) {
+        private fun add() {
+            if (count <= 0) {
+                return
+            }
+
+            compressed.add(count to character)
+        }
+
+        fun update(c: Char): State {
+            if (c != character || count >= 9) {
+                add()
+                return State(compressed, 1, c)
+            }
+
+            return State(compressed, 1 + count, c)
+        }
+
+        fun finalize(): State {
+            add()
+            return State(compressed, 0, '_')
+        }
+    }
+
+    fun compressedString(word: String): String =
+        word.fold(State(mutableListOf(), 0, word[0])) { acc, it ->
+            acc.update(it)
+        }.finalize().compressed.joinToString(separator = "") {
+            "${it.first}${it.second}"
+        }
+}