Add Quick Sort algorithm implementation

anverbogatov · anverbogatov · commit 2d7858dca2ed · 2022-03-21T20:35:25.000+04:00
diff --git a/src/main/java/com/anverbogatov/algorithms/sorting/QuickSort.java b/src/main/java/com/anverbogatov/algorithms/sorting/QuickSort.java
@@ -0,0 +1,41 @@
+package com.anverbogatov.algorithms.sorting;
+
+public class QuickSort {
+    /**
+     * Sort given array of numbers using Quick Sort algorithm.
+     * The algorithm has complexity equals O(n * log n).
+     *
+     * @param numbers - array of numbers that need to be sorted
+     */
+    public static void sort(int[] numbers) {
+        if (numbers.length < 2) return;
+        sort(numbers, 0, numbers.length - 1);
+    }
+
+    private static void sort(int[] numbers, int low, int high) {
+        if (high <= low) return;
+        var pivot = partition(numbers, low, high);
+        sort(numbers, low, pivot - 1);
+        sort(numbers, pivot + 1, high);
+    }
+
+    private static int partition(int[] numbers, int low, int high) {
+        var i = low;
+        var j = high + 1;
+        var pivotEl = numbers[low];
+        while (true) {
+            while (numbers[++i] < pivotEl) if (i == high) break;
+            while (pivotEl < numbers[--j]) if (j == low) break;
+            if (i >= j) break;
+            swap(numbers, i, j);
+        }
+        swap(numbers, low, j);
+        return j;
+    }
+
+    private static void swap(int[] numbers, int i, int j) {
+        var temp = numbers[i];
+        numbers[i] = numbers[j];
+        numbers[j] = temp;
+    }
+}
diff --git a/src/test/java/com/anverbogatov/algorithms/sorting/QuickSortTest.java b/src/test/java/com/anverbogatov/algorithms/sorting/QuickSortTest.java
@@ -0,0 +1,40 @@
+package com.anverbogatov.algorithms.sorting;
+
+import org.junit.Test;
+
+import java.util.Arrays;
+
+import static org.junit.Assert.assertArrayEquals;
+
+public class QuickSortTest {
+    @Test
+    public void sortEmpty() {
+        // given
+        var numbers = new int[]{};
+        // when
+        QuickSort.sort(numbers);
+        // then
+        assertArrayEquals(new int[]{}, numbers);
+    }
+
+    @Test
+    public void sortSingle() {
+        // given
+        var numbers = new int[]{1};
+        // when
+        QuickSort.sort(numbers);
+        // then
+        assertArrayEquals(new int[]{1}, numbers);
+    }
+
+    @Test
+    public void sort() {
+        // given
+        var numbers = new int[]{10, 15, 3, 7, 9, 2, 4, 5};
+        // when
+        QuickSort.sort(numbers);
+        // then
+        System.out.println("[Quick sort]: = " + Arrays.toString(numbers));
+        assertArrayEquals(new int[]{2, 3, 4, 5, 7, 9, 10, 15}, numbers);
+    }
+}
