From 999dc93f2397e954360043fe5be21b3442ca7a1c Mon Sep 17 00:00:00 2001
From: lq <liuqian920320@163.com>
Date: Sun, 9 Sep 2018 20:14:32 +0800
Subject: [PATCH] add MergeSortTest for MergeSort.

---
 src/main/java/com/sorts/MergeSort.java     | 75 ++++++++++++++++++++++
 src/test/java/com/sorts/MergeSortTest.java | 34 ++++++++++
 2 files changed, 109 insertions(+)
 create mode 100644 src/main/java/com/sorts/MergeSort.java
 create mode 100644 src/test/java/com/sorts/MergeSortTest.java

diff --git a/src/main/java/com/sorts/MergeSort.java b/src/main/java/com/sorts/MergeSort.java
new file mode 100644
index 00000000..3582a08b
--- /dev/null
+++ b/src/main/java/com/sorts/MergeSort.java
@@ -0,0 +1,75 @@
+package src.main.java.com.sorts;
+
+public class MergeSort {
+
+  /**
+   * This method implements the Generic Merge Sort
+   *
+   * @param unsorted the array which should be sorted
+   * @param <T>      Comparable class
+   * @return sorted array
+   */
+  @SuppressWarnings("unchecked")
+  public <T extends Comparable<T>> T[] sort(T[] unsorted) {
+    T[] tmp = (T[]) new Comparable[unsorted.length];
+    doSort(unsorted, tmp, 0, unsorted.length - 1);
+    return unsorted;
+  }
+
+  /**
+   * @param arr   The array to be sorted
+   * @param temp  The copy of the actual array
+   * @param left  The first index of the array
+   * @param right The last index of the array
+   *              Recursively sorts the array in increasing order
+   **/
+  private static <T extends Comparable<T>> void doSort(T[] arr, T[] temp, int left, int right) {
+    if (left < right) {
+      int mid = left + (right - left) / 2;
+      doSort(arr, temp, left, mid);
+      doSort(arr, temp, mid + 1, right);
+      merge(arr, temp, left, mid, right);
+    }
+  }
+
+  /**
+   * This method implements the merge step of the merge sort
+   *
+   * @param arr   The array to be sorted
+   * @param temp  The copy of the actual array
+   * @param left  The first index of the array
+   * @param mid   The middle index of the array
+   * @param right The last index of the array
+   *              merges two parts of an array in increasing order
+   **/
+  private static <T extends Comparable<T>> void merge(T[] arr, T[] temp, int left, int mid, int right) {
+    System.arraycopy(arr, left, temp, left, right - left + 1);
+
+    int i = left;
+    int j = mid + 1;
+    int k = left;
+
+    while (i <= mid && j <= right) {
+      if (temp[i].compareTo(temp[j]) <= 0) {
+        arr[k] = temp[i];
+        i++;
+      } else {
+        arr[k] = temp[j];
+        j++;
+      }
+      k++;
+    }
+
+    while (i <= mid) {
+      arr[k] = temp[i];
+      i++;
+      k++;
+    }
+
+    while (j <= right) {
+      arr[k] = temp[j];
+      j++;
+      k++;
+    }
+  }
+}
diff --git a/src/test/java/com/sorts/MergeSortTest.java b/src/test/java/com/sorts/MergeSortTest.java
new file mode 100644
index 00000000..25851523
--- /dev/null
+++ b/src/test/java/com/sorts/MergeSortTest.java
@@ -0,0 +1,34 @@
+package src.test.java.com.sorts;
+
+
+import org.junit.Assert;
+import org.junit.Test;
+import src.main.java.com.sorts.MergeSort;
+
+public class MergeSortTest {
+
+  @Test
+  public void mergeSortTest() {
+    MergeSort mergeSort = new MergeSort();
+
+    Integer[] unsortedInt = new Integer[]{0, 5, 9, 2, 1, 3, 4, 8, 6, 7};
+    Integer[] sortedInt = new Integer[]{0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
+    Assert.assertArrayEquals(sortedInt, mergeSort.sort(unsortedInt));
+
+    unsortedInt = new Integer[]{5, 4, 3, 2, 1, 0};
+    sortedInt = new Integer[]{0, 1, 2, 3, 4, 5};
+    Assert.assertArrayEquals(sortedInt, mergeSort.sort(unsortedInt));
+
+    unsortedInt = new Integer[]{-1, -2, -3, -4, -5};
+    sortedInt = new Integer[]{-5, -4, -3, -2, -1};
+    Assert.assertArrayEquals(sortedInt, mergeSort.sort(unsortedInt));
+
+    unsortedInt = new Integer[]{-1, -5, -10, -990, 990, 1010};
+    sortedInt = new Integer[]{-990, -10, -5, -1, 990, 1010};
+    Assert.assertArrayEquals(sortedInt, mergeSort.sort(unsortedInt));
+
+    Character[] unsortedChar = new Character[]{'f', 'h', 'c', 'a', 'b', 'd', 'g', 'e'};
+    Character[] sortedChar = new Character[]{'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h'};
+    Assert.assertArrayEquals(sortedChar, mergeSort.sort(unsortedChar));
+  }
+}