Added quick select. (williamfiset#272)

Co-authored-by: Saruul Shafiq <[email protected]>

Author: jasonsaruulo

src/main/java/com/williamfiset/algorithms/sorting/QuickSelect.java

@@ -0,0 +1,57 @@
+package com.williamfiset.algorithms.sorting;
+
+public class QuickSelect {
+
+    public Integer quickSelect(int[] ar, int k) {
+        if (ar == null) return null;
+        if (k > ar.length) return null;
+        if (k < 1) return null;
+        return quickSelect(ar, k, 0, ar.length - 1);
+    }
+
+    // Sort interval [lo, hi] inplace recursively, returns value when splitPoint == k - 1
+    private static Integer quickSelect(int[] ar, int k, int lo, int hi) {
+        int index = k - 1;
+        if (lo < hi) {
+            int splitPoint = partition(ar, lo, hi);
+            if (splitPoint == index) {
+                return ar[splitPoint];
+            } else if (splitPoint > index) {
+                return quickSelect(ar, k, lo, splitPoint);
+            }
+            return quickSelect(ar, k, splitPoint + 1, hi);
+        }
+        return ar[lo];
+    }
+
+    // Performs Hoare partition algorithm for quick select, taken from QuickSelect implementation
+    private static int partition(int[] ar, int lo, int hi) {
+        int pivot = ar[lo];
+        int i = lo - 1, j = hi + 1;
+        while (true) {
+            do {
+                i++;
+            } while (ar[i] < pivot);
+            do {
+                j--;
+            } while (ar[j] > pivot);
+            if (i < j) swap(ar, i, j);
+            else return j;
+        }
+    }
+
+    // Swap two elements
+    private static void swap(int[] ar, int i, int j) {
+        int tmp = ar[i];
+        ar[i] = ar[j];
+        ar[j] = tmp;
+    }
+
+    public static void main(String[] args) {
+        QuickSelect quickSelect = new QuickSelect();
+        int[] array = {-10, 4, 6, 4, 8, -13, 1, 3};
+        int kthLargestElement = quickSelect.quickSelect(array, 3);
+        // Prints: 1
+        System.out.println(kthLargestElement);
+    }
+}

src/test/java/com/williamfiset/algorithms/sorting/QuickSelectTest.java

@@ -0,0 +1,31 @@
+package com.williamfiset.algorithms.sorting;
+
+import com.williamfiset.algorithms.utils.TestUtils;
+import org.junit.Test;
+
+import java.util.Arrays;
+
+import static com.google.common.truth.Truth.assertThat;
+
+public class QuickSelectTest {
+
+    @Test
+    public void testQuickSelect() {
+        for (int size = 1; size < 500; size++) {
+            // Given
+            QuickSelect quickSelect = new QuickSelect();
+            int[] values = TestUtils.randomIntegerArray(size, -100, 100);
+
+            for (int k = 1; k <= size; k++) {
+                int[] copy = values.clone();
+                Arrays.sort(values);
+
+                // When
+                int kthLargestElement = quickSelect.quickSelect(copy, k);
+
+                // Then
+                assertThat(kthLargestElement).isEqualTo(values[k - 1]);
+            }
+        }
+    }
+}