0.时间复杂度
lgn * n

1.Java实现
    private static void println(int[] array) {
        for (int item : array) {
            System.out.print(item + " ");
        }
        System.out.println();
    }

    private static void mergeSortByIndex(int[] array, int startIndex, int endIndex) {
        // 最小处理块1 然后依次往上层归并
        if (startIndex < endIndex) {
            int centerIndex = (startIndex + endIndex) / 2;
            mergeSortByIndex(array, startIndex, centerIndex);
            mergeSortByIndex(array, centerIndex + 1, endIndex);
            mergeArray(array, startIndex, centerIndex, endIndex);
        }
    }

    private static void mergeArray(int[] array, int startIndex, int centerIndex, int endIndex) {
        int[] tempArray = new int[endIndex - startIndex + 1];
        int leftIndex = startIndex;
        int rightIndex = centerIndex + 1;
        boolean leftFlag = true;
        boolean rightFlag = true;
        int temp = 0;
        while (temp <= endIndex - startIndex) {
            if (leftIndex > centerIndex) {
                leftFlag = false;
            }
            if (rightIndex > endIndex) {
                rightFlag = false;
            }
            if (leftFlag && rightFlag) {
                // 左右2边都有
//                if (array[leftIndex] <= array[rightIndex]) {
                if (array[leftIndex] >= array[rightIndex]) {
                    tempArray[temp] = array[leftIndex];
                    leftIndex ++;
                } else {
                    tempArray[temp] = array[rightIndex];
                    rightIndex ++;
                }
            } else if (!leftFlag && !rightFlag) {
                // 左右2边都没有
                break;
            } else if (!leftFlag && rightFlag) {
                // 只有右边有
                tempArray[temp] = array[rightIndex];
                rightIndex ++;
            } else {
                // 只有左边有
                tempArray[temp] = array[leftIndex];
                leftIndex ++;
            }
            temp ++;
        }

        // 复制到原数组中
        for (int copyIndex  = 0; copyIndex  < tempArray.length; copyIndex ++) {
            array[startIndex + copyIndex] = tempArray[copyIndex];
        }
    }

    private static void mergeSort(int[] array) {
        mergeSortByIndex(array, 0, array.length - 1);
    }

    public static void main(String[] args) throws Exception {
//        int[] array = {10, 9, 8};
        int[] array = {8, 9, 10};
        println(array);
        mergeSort(array);
        println(array);
    }