java归并排序

private void mergeSort(int[] array, int begin, int end) {
        if (begin >= end || begin < 0 || end > array.length - 1) {
            return;
        }
        int mid = begin + (end - begin) / 2;
        mergeSort(array, begin, mid);
        mergeSort(array, mid + 1, end);
        merge(array, begin, mid, end);
 }

 private void merge(int[] array, int begin, int mid, int end) {
        int b1 = begin;
        int b2 = mid + 1;
        int length = end - b1 + 1;
        int[] tmp = new int[length];
        int index;
        for (index = 0; index < length && b1 <= mid && b2 <= end; index++) {
            if (array[b1] > array[b2]) {
                tmp[index] = array[b2];
                b2++;
            } else {
                tmp[index] = array[b1];
                b1++;
            }
        }
        if (b1 > mid) {
            for (; index < length; index++) {
                tmp[index] = array[b2];
                b2++;
            }
        } else {
            for (; index < length; index++) {
                tmp[index] = array[b1];
                b1++;
            }
        }

        System.arraycopy(tmp, 0, array, begin, length);
}