以下代码采用分而治之算法实现归并排序。请补充函数mergesort（）的代码。提示：mergesort（）函数可用递归实现，其中参数n在递归调用中不需要变化。 函数接口定义： void mergesort(int a[],int n,int left,int right)；

void merge(int a[], int left, int mid, int right) {
    int i = left;
    int j = mid + 1;
    int k = 0;
    int* tmp = new int[right - left + 1];
    while (i <= mid && j <= right) {
        if (a[i] <= a[j]) {
            tmp[k++] = a[i++];
        }
        else {
            tmp[k++] = a[j++];
        }
    }
    while (i <= mid) {
        tmp[k++] = a[i++];
    }
    while (j <= right) {
        tmp[k++] = a[j++];
    }
    for (int x = 0; x < k; x++) {
        a[left + x] = tmp[x];
    }
    delete[] tmp;
}

void mergesort(int a[], int n, int left, int right) {
    if (left < right) {
        int mid = (left + right) / 2;
        mergesort(a, n, left, mid);
        mergesort(a, n, mid + 1, right);
        merge(a, left, mid, right);
    }
}