MergeSort(Java)

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));
    }
}