把数组分成两半，递归地排好每一半，合并有序的两半
稳定排序 
需要辅助空间 非就地排序
归并排序之父 冯诺依曼



public class MergeSort {
    public static void main(String[] args) throws Exception {
        int[] arr = { 3, 2, 1, 9, 4, 8, 7, 5, 6 };
        mergeSort(arr);
        printArr(arr);
    }

    public static void mergeSort(int[] a) {
        if (a == null || a.length <= 1) {
            return;
        }
        // a.length - 1 !!!
        mergeSort(a, 0, a.length - 1);
    }

    public static void mergeSort(int[] a, int low, int high) {

        int mid = (low + high) / 2;
        if (low < high) {
            mergeSort(a, low, mid);
            mergeSort(a, mid + 1, high);
            merge(a, low, mid, high);
        }
    }

    private static void merge(int[] a, int low, int mid, int high) {
        int[] temp = new int[high - low + 1];
        int i = low;
        int j = mid + 1;
        int k = 0;

        while (i <= mid && j <= high) {
            if (a[i] < a[j]) {
                temp[k++] = a[i++];
            } else {
                temp[k++] = a[j++];
            }
        }
        // 左边剩下的数放入数组
        while (i <= mid) {
            temp[k++] = a[i++];
        }
        // 右边剩下的数放入数组
        while (j <= high) {
            temp[k++] = a[j++];
        }
        //
        for (int x = 0; x < temp.length; x++) {
            a[x + low] = temp[x];
        }
    }

    private static void printArr(int[] arr) {
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < arr.length; i++) {
            sb.append(arr[i]).append(" ");
        }
        System.out.println(sb.toString());
    }
}


https://gitee.com/dyyx/hellocode/blob/master/src/algo/sort/MergeSort.java



 归并排序与快速排序