Add Ex 23.02

Author: Harry Dulaney

ch_23/Exercise23_02.java

@@ -0,0 +1,90 @@
+package ch_23;
+
+import java.util.Arrays;
+import java.util.Comparator;
+
+/**
+ * 23.2 (Generic merge sort) Write the following two generic methods using merge sort.
+ * The first method sorts the elements using the Comparable interface and the
+ * second uses the Comparator interface.
+ * public static <E extends Comparable<E>>
+ * void mergeSort(E[] list)
+ * public static <E> void mergeSort(E[] list,
+ * Comparator<? super E> comparator)
+ */
+public class Exercise23_02 {
+    public static void main(String[] args) {
+        Integer[] arr = new Integer[]{23, 25, 7, 14, 85, 65};
+        mergeSort(arr);
+        System.out.println(Arrays.toString(arr));
+
+        Integer[] arr2 = new Integer[]{3, 2, 7, 4, 8, 6, 1, 7};
+        mergeSort(arr2, Integer::compareTo);
+        System.out.println(Arrays.toString(arr2));
+
+    }
+
+    public static <E> void mergeSort(E[] list,
+                                     Comparator<? super E> comparator) {
+        if (list.length > 1) {
+            E[] firstHalf = Arrays.copyOf(list, list.length / 2);
+            System.arraycopy(list, 0, firstHalf, 0, list.length / 2);
+            mergeSort(firstHalf, comparator);
+
+            int secondHalfLength = list.length - list.length / 2;
+            E[] secondHalf = Arrays.copyOf(list, secondHalfLength);
+            System.arraycopy(list, list.length / 2,
+                    secondHalf, 0, secondHalfLength);
+            mergeSort(secondHalf, comparator);
+
+            int current1, current2, current3;
+            current1 = current2 = current3 = 0;
+
+            while (current1 < firstHalf.length && current2 < secondHalf.length) {
+                if (comparator.compare(firstHalf[current1], secondHalf[current2]) < 0)
+                    list[current3++] = firstHalf[current1++];
+                else
+                    list[current3++] = secondHalf[current2++];
+            }
+
+            while (current1 < firstHalf.length)
+                list[current3++] = firstHalf[current1++];
+
+            while (current2 < secondHalf.length)
+                list[current3++] = secondHalf[current2++];
+        }
+    }
+
+
+    public static <E extends Comparable<E>> void mergeSort(E[] list) {
+        if (list.length > 1) {
+
+            E[] firstHalf = Arrays.copyOf(list, list.length / 2);
+            System.arraycopy(list, 0, firstHalf, 0, list.length / 2);
+            mergeSort(firstHalf);
+
+            int secondHalfLength = list.length - list.length / 2;
+            E[] secondHalf = Arrays.copyOf(list, secondHalfLength);
+            System.arraycopy(list, list.length / 2,
+                    secondHalf, 0, secondHalfLength);
+            mergeSort(secondHalf);
+
+            int current1, current2, current3;
+            current1 = current2 = current3 = 0;
+
+            while (current1 < firstHalf.length && current2 < secondHalf.length) {
+                if (firstHalf[current1].compareTo(secondHalf[current2]) < 0)
+                    list[current3++] = firstHalf[current1++];
+                else
+                    list[current3++] = secondHalf[current2++];
+            }
+
+            while (current1 < firstHalf.length)
+                list[current3++] = firstHalf[current1++];
+
+            while (current2 < secondHalf.length)
+                list[current3++] = secondHalf[current2++];
+        }
+    }
+
+}