add another solution for dutch flag,

add swap extension function to arrays

Author: abdurahmanadilovic

.idea/workspace.xml

@@ -1,5 +1,7 @@
 package ch6Arrays
 
+import ch5PrimitiveTypes.swap
+
 fun main(args: Array<String>) {
     solution1(arrayOf(3, 5, 4, 6, 7, 1, 2), 2).forEach {
         print(it)
@@ -16,6 +18,12 @@ fun main(args: Array<String>) {
     solution3(arrayOf(3, 5, 4, 6, 7, 1, 2), 2).forEach {
         print(it)
     }
+
+    println()
+
+    solution4(arrayOf(3, 5, 4, 6, 7, 1, 2), 2).forEach {
+        print(it)
+    }
 }
 
 // running time O(n), space complexity 0(n)
@@ -61,9 +69,7 @@ fun solution2(numbers: Array<Int>, pivotPosition: Int): Array<Int> {
     for (i in 0..numbers.size) {
         for (j in i + 1 until numbers.size) {
             if (numbers[j] < pivot) {
-                val temp = numbers[i]
-                numbers[i] = numbers[j]
-                numbers[j] = temp
+                numbers.swap(i, j)
             }
         }
     }
@@ -72,9 +78,7 @@ fun solution2(numbers: Array<Int>, pivotPosition: Int): Array<Int> {
     for (i in numbers.size-1 downTo 0) {
         for (j in i - 1 downTo 0) {
             if (numbers[j] > pivot) {
-                val temp = numbers[i]
-                numbers[i] = numbers[j]
-                numbers[j] = temp
+                numbers.swap(i, j)
             }
         }
     }
@@ -90,21 +94,44 @@ fun solution3(numbers: Array<Int>, pivotPosition: Int): Array<Int> {
 
     for(i in 0 until numbers.size){
         if(numbers[i] < pivot){
-            val temp = numbers[leftIndex]
-            numbers[leftIndex]  = numbers[i]
-            numbers[i] = temp
-            leftIndex++
+            numbers.swap(leftIndex++, i)
         }
     }
 
     for(i in numbers.size-1 downTo 0){
         if(numbers[i] > pivot){
-            val temp = numbers[rightIndex]
-            numbers[rightIndex]  = numbers[i]
-            numbers[i] = temp
-            rightIndex--
+            numbers.swap(rightIndex--, i)
         }
     }
 
     return numbers
 }
+
+/*
+ The idea here is that we mark regions, smaller than pivot on the left,
+ then the pivot, then unclassified elements left from the result of previous swapping
+ then bigger elements
+ */
+fun solution4(numbers: Array<Int>, pivotPosition: Int): Array<Int> {
+    var smaller = 0
+    var equal = 0
+    val pivot = numbers[pivotPosition]
+    var bigger = numbers.size - 1
+
+    // while we have unclassified elements until all are classified iterate
+    while(equal < bigger){
+        when {
+            numbers[equal] < pivot -> numbers.swap(equal++, smaller++)
+            numbers[equal] > pivot -> numbers.swap(bigger--, equal)
+            numbers[equal] == pivot -> equal++
+        }
+    }
+
+    return numbers
+}
+
+fun Array<Int>.swap(a: Int, b: Int){
+    val tmp = this[a]
+    this[a] = this[b]
+    this[b] = tmp
+}