03 & 04 exercises (#96)

03_recursion/kotlin/01_countdown.kt

@@ -0,0 +1,11 @@
+private fun countdown(i: Int) {
+    println(i)
+    when {
+        // base case
+        i <= 0 -> return
+        // recursive case
+        else -> countdown(i - 1)
+    }
+}
+
+fun main(args: Array<String>) = countdown(5)

03_recursion/kotlin/02_greet.kt

@@ -0,0 +1,12 @@
+private fun greet2(name: String) = println("how are you, $name?")
+
+private fun bye() = println("ok bye!")
+
+private fun greet(name: String) {
+    println("hello, $name!")
+    greet2(name)
+    println("getting ready to say bye...")
+    bye()
+}
+
+fun main(args: Array<String>) = greet("adit")

03_recursion/kotlin/03_factorial.kt

@@ -0,0 +1,6 @@
+private fun fact(x: Int): Int = when (x) {
+    1 -> 1
+    else -> x * fact(x - 1)
+}
+
+fun main(args: Array<String>) = println(fact(5))

04_quicksort/kotlin/01_loop_sum.kt

@@ -0,0 +1,7 @@
+private fun sum(arr: Array<Int>): Int {
+    var total = 0
+    for (x in arr) total += x
+    return total
+}
+
+fun main(args: Array<String>) = println(sum(arrayOf(1, 2, 3, 4)))

04_quicksort/kotlin/02_recursive_sum.kt

@@ -0,0 +1,6 @@
+private fun sum(arr: IntArray): Int = when {
+    arr.isEmpty() -> 0
+    else -> arr[0] + sum(arr.copyOfRange(1, arr.size))
+}
+
+fun main(args: Array<String>) = println(sum(intArrayOf(1, 2, 3, 4)))

04_quicksort/kotlin/03_recursive_count.kt

@@ -0,0 +1,6 @@
+private fun count(list: List<Any>): Int = when {
+    list.isEmpty() -> 0
+    else -> 1 + count(list.subList(1, list.size))
+}
+
+fun main(args: Array<String>) = println(count(listOf(1, 2, 3, 4, 5)))

04_quicksort/kotlin/04_recursive_max.kt

@@ -0,0 +1,9 @@
+private fun max(list: IntArray): Int = when {
+    list.size == 2 -> if (list[0] > list[1]) list[0] else list[1]
+    else -> {
+        val subMax = max(list.copyOfRange(1, list.size))
+        if (list[0] > subMax) list[0] else subMax
+    }
+}
+
+fun main(args: Array<String>) = println(max(intArrayOf(1, 5, 10, 25, 16, 1))) // 25

04_quicksort/kotlin/05_quicksort.kt

@@ -0,0 +1,12 @@
+fun quickSort(list: List<Int>): List<Int> {
+    // base case, arrays with 0 or 1 element are already "sorted"
+    if (list.size <= 1) return list
+    // recursive case
+    val pivot = list[list.size / 2]
+    val equal = list.filter { it == pivot }
+    val less = list.filter { it < pivot }
+    val greater = list.filter { it > pivot }
+    return quickSort(less) + equal + quickSort(greater)
+}
+
+fun main(args: Array<String>) = println(quickSort(listOf(10, 5, 2, 3))) // [2, 3, 5, 10]