solution: fot Theme "rs.android-2021-stage1-task1"

src/main/kotlin/subtask1/HappyArray.kt

@@ -2,8 +2,17 @@ package subtask1
 
 class HappyArray {
 
-    // TODO: Complete the following function
     fun convertToHappy(sadArray: IntArray): IntArray {
-        throw NotImplementedError("Not implemented")
+        var i = 1
+        var happyList = sadArray.toMutableList()
+        while (i < happyList.size - 1) {
+            if ( i != 0 && happyList[i] > happyList[i + 1] + happyList[i - 1]) {
+                happyList.removeAt(i)
+                i --
+            } else {
+                i ++
+            }
+        }
+        return happyList.toIntArray()
     }
 }

src/main/kotlin/subtask2/BillCounter.kt

@@ -2,9 +2,12 @@ package subtask2
 
 class BillCounter {
 
-    // TODO: Complete the following function
-    // The output could be "Bon Appetit" or the string with number(e.g "10")
     fun calculateFairlySplit(bill: IntArray, k: Int, b: Int): String {
-        throw NotImplementedError("Not implemented")
+        val sum = (bill.sum() - bill[k]) / 2
+        return if (sum - b  == 0) {
+            "Bon Appetit"
+        } else {
+            (b - sum).toString()
+        }
     }
 }

src/main/kotlin/subtask3/StringParser.kt

@@ -2,8 +2,28 @@ package subtask3
 
 class StringParser {
 
-    // TODO: Complete the following function
     fun getResult(inputString: String): Array<String> {
-        throw NotImplementedError("Not implemented")
+        val result: MutableList<String> = mutableListOf()
+        inputString.forEachIndexed { index, s ->
+            when (s) {
+                '(' -> result.add(getSubString(inputString, index, '(', ')'))
+                '[' -> result.add(getSubString(inputString, index, '[', ']'))
+                '<' -> result.add(getSubString(inputString, index, '<', '>'))
+            }
+        }
+        return result.toTypedArray()
+    }
+
+    private fun getSubString(inputString: String, index: Int, openingChar: Char, closingChar: Char): String {
+        var i = index
+        var n = 0
+        while (n < 1) {
+            i++
+            if (inputString[i] == openingChar)
+                n--
+            if (inputString[i] == closingChar)
+                n++
+        }
+        return (inputString.substring(index + 1, i))
     }
 }