实现代码：MergeSort.java

public class MergeSort {

public int[] sort(int[] input) {

if (input.length <= 1) return input;

if (input.length == 2) {

if (input[0] > input[1]) {

int temp = input[0];

input[0] = input[1];

input[1] = temp;

}

return input;

}

int mid = input.length / 2;

int[] firstHalf = sort(getPart(input, 0, mid));

int[] secondHalf = sort(getPart(input, mid + 1, input.length - 1));

return mergePart(firstHalf, secondHalf);

}

private int[] mergePart(int[] firstHalf, int[] secondHalf) {

int[] result = new int[firstHalf.length + secondHalf.length];

int firstIndex = 0, secondIndex = 0, resultIndex = 0;

while (resultIndex < result.length) {

if (chooseFirstHalf(firstHalf, firstIndex, secondHalf, secondIndex))

result[resultIndex++] = firstHalf[firstIndex++];

else

result[resultIndex++] = secondHalf[secondIndex++];

}

return result;

}

private boolean chooseFirstHalf(int[] firstHalf, int firstIndex, int[] secondHalf, int secondIndex) {

if (firstIndex == firstHalf.length) return false;

if (secondIndex == secondHalf.length) return true;

return firstHalf[firstIndex] < secondHalf[secondIndex];

}

private int[] getPart(int[] input, int begin, int end) {

int[] result = new int[end - begin + 1];

for (int i = begin; i <= end; i++) result[i - begin] = input[i];

return result;

}

}

测试代码：MergeSortTest.java

import org.junit.*;

import static org.junit.Assert.*;

public class MergeSortTest {

MergeSort mergeSort;

@Before

public void setUp() {

mergeSort = new MergeSort();

}

@Test

public void should_return_1_for_merge_sort_1() {

int[] input = {1};

int[] expected = {1};

assertArrayEquals(expected, mergeSort.sort(input));

}

@Test

public void should_return_12_for_merge_sort_21() {

int[] input = {2,1};

int[] expected = {1,2};

assertArrayEquals(expected, mergeSort.sort(input));

}

@Test

public void should_return_1234_for_merge_sort_3214() {

int[] input = {3,2,1,4};

int[] expected = {1,2,3,4};

assertArrayEquals(expected, mergeSort.sort(input));

}

@Test

public void should_return_12345_for_merge_sort_54321() {

int[] input = {5, 4, 3, 2, 1};

int[] expected = {1, 2, 3, 4, 5};

assertArrayEquals(expected, mergeSort.sort(input));

}

}