Bael 5723 gravity sort and unit test (#12891)

* add Gravity Sort java class

* gravity sort in java with unit test

* Update GravitySort.java

fix name in setupAbacus method

* Update GravitySortUnitTest.java

fix name in unit test method

Co-authored-by: Christian Jaimes <christian.jaimes@oracle>

algorithms-modules/algorithms-sorting-2/src/main/java/com/baeldung/algorithms/gravitysort/GravitySort.java

@@ -0,0 +1,60 @@
+package com.baeldung.algorithms.gravitysort;
+
+public class GravitySort {
+
+    public static int findMax(int[] A) {
+        int max = A[0];
+        for (int i = 1; i< A.length; i++) {
+            if (A[i] > max) {
+                max = A[i];
+            }
+        }
+        return max;
+    }
+
+    public static boolean[][] setupAbacus(int[] A, int m) {
+        boolean[][] abacus = new boolean[A.length][m];
+        for (int i = 0; i < abacus.length; i++) {
+            int number = A[i];
+            for (int j = 0; j < abacus[0].length && j < number; j++) {
+                abacus[A.length - 1 - i][j] = true;
+            }
+        }
+        return abacus;
+    }
+
+    public static void dropBeads(boolean[][] abacus, int[] A, int m) {
+        for (int i = 1; i < A.length; i++) {
+            for (int j = m - 1; j >= 0; j--) {
+                if (abacus[i][j] == true) {
+                    int x = i;
+                    while (x > 0 && abacus[x - 1][j] == false) {
+                        boolean temp = abacus[x - 1][j];
+                        abacus[x - 1][j] = abacus[x][j];
+                        abacus[x][j] = temp;
+                        x--;
+                    }
+                }
+            }
+        }
+    }
+
+    public static void transformToList(boolean[][] abacus, int[] A) {
+        int index = 0;
+        for (int i = abacus.length - 1; i >= 0; i--) {
+            int beads = 0;
+            for (int j = 0; j < abacus[0].length && abacus[i][j] == true; j++) {
+                beads++;
+            }
+            A[index++] = beads;
+        }
+    }
+
+    public static void sort(int[] A) {
+        int m = findMax(A);
+        boolean[][] abacus = setupAbacus(A, m);
+        dropBeads(abacus, A, m);
+        // transform abacus into sorted list
+        transformToList(abacus, A);
+    }
+}

algorithms-modules/algorithms-sorting-2/src/test/java/com/baeldung/algorithms/gravitysort/GravitySortUnitTest.java

@@ -0,0 +1,15 @@
+package com.baeldung.algorithms.gravitysort;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+public class GravitySortUnitTest {
+
+    @Test
+    public void givenIntegerArray_whenSortedWithGravitySort_thenGetSortedArray() {
+        int[] actual = { 9, 9, 100, 3, 57, 12, 3, 78, 0, 2, 2, 40, 21, 9 };
+        int[] expected = { 0, 2, 2, 3, 3, 9, 9, 9, 12, 21, 40, 57, 78, 100 };
+        GravitySort.sort(actual);
+        Assert.assertArrayEquals(expected, actual);
+    }
+}