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));
}
}
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