tp8 step 3 solution

Author: ctruchi

tp8/src/main/kotlin/fmt/kotlin/fundamentals/MapExt.kt

@@ -9,4 +9,11 @@ package fmt.kotlin.fundamentals
  * @param operation an operation to call on entry with a value in both map to compute the new value
  * @return a new map with keys of both maps and values computed with [operation]
  */
-fun Map<String, Int>.merge(map: Map<String, Int>, operation: (Int, Int) -> Int): Map<String, Int> = TODO()
+fun Map<String, Int>.merge(map: Map<String, Int>, operation: (Int, Int) -> Int): Map<String, Int> =
+    (keys + map.keys).associateWith { key ->
+        if (this[key] != null && map[key] != null) {
+            operation(this[key]!!, map[key]!!)
+        } else {
+            this[key] ?: map[key]!!
+        }
+    }