public static void mergeSort(int[] a, int[] tmp, int left, int right) {
    if (left < right) {
      int mid = left + (right - left) / 2;
      mergeSort(a, tmp, left, mid);// 左排序
      mergeSort(a, tmp, mid + 1, right);// 右排序
      merge(a, tmp, left, mid + 1, right);// 左右合并
    }
  }
public static void merge(int[] a, int[] tmp, int left, int rightPos,
      int right) {
    int leftEnd = rightPos - 1;
    int tmpPos = left;
    int num = right - left + 1;
    while (left <= leftEnd && rightPos <= right) {
      if (a[left] < a[rightPos]) {
        tmp[tmpPos++] = a[left++];
      } else {
        tmp[tmpPos++] = a[rightPos++];
      }
    }
    while (left <= leftEnd) {
      tmp[tmpPos++] = a[left++];
    }
    while (rightPos <= right) {
      tmp[tmpPos++] = a[rightPos++];
    }
    for (int i = 0; i < num; i++, right--) {
      a[right] = tmp[right];
    }
  }