add Bottom-up Merge sort impl

src/main/java/algorithms/sorting/MergeBU.java

@@ -0,0 +1,76 @@
+package algorithms.sorting;
+
+import static algorithms.Assert.assertEquals;
+import static algorithms.sorting.SortUtils.isSorted;
+import static algorithms.sorting.SortUtils.print;
+import static algorithms.sorting.SortUtils.referenceSort;
+import static java.lang.Math.min;
+import static java.util.Arrays.copyOf;
+
+/**
+ * Bottom-up Merge sort algorithm without recursion.
+ * As reference used:
+ * @see <a href="https://algs4.cs.princeton.edu/code/edu/princeton/cs/algs4/MergeBU.java.html>
+ */
+public class MergeBU {
+
+    public static void sort(int[] a) {
+
+        int n = a.length;
+        int[] aux = new int[n];
+
+        for (int sz = 1; sz < n; sz *= 2) {
+            for (int lo = 0; lo < n - sz; lo += 2 * sz) {
+                int mid = lo + sz - 1;
+                int hi = min(lo + 2 * sz - 1, n - 1);
+                merge(a, aux, lo, mid, hi);
+            }
+        }
+
+        assert isSorted(a);
+    }
+
+    private static void merge(int[] a, int[] aux, int lo, int mid, int hi) {
+
+        assert isSorted(a, lo, mid);
+        assert isSorted(a, mid + 1, hi);
+
+        for (int k = lo; k <= hi; k++) {
+            aux[k] = a[k];
+        }
+
+        int i = lo, j = mid + 1;
+        for (int k = lo; k <= hi; k++) {
+
+            if (i > mid)         a[k] = aux[j++];
+            else if (j > hi)          a[k] = aux[i++];
+            else if (aux[j] < aux[i]) a[k] = aux[j++];
+            else                      a[k] = aux[i++];
+        }
+
+        assert isSorted(a, lo, hi);
+    }
+
+    public static void main(String[] args) {
+        int[] array = {2, 5, 5, 1, 6, 0, 4, 3};
+        int[] copy  = copyOf(array, array.length);
+
+        sort(array);
+        print(array);
+        assertEquals(array, referenceSort(copy));
+
+        array = new int[]{6, 5, 4, 3, 2, 1, 0};
+        copy  = copyOf(array, array.length);
+
+        sort(array);
+        print(array);
+        assertEquals(array, referenceSort(copy));
+
+        array = new int[]{1, 1, 2, 2, 3, 4};
+        copy  = copyOf(array, array.length);
+
+        sort(array);
+        print(array);
+        assertEquals(array, referenceSort(copy));
+    }
+}