采用分治思想

1.划分子区间

2.递归处理子区间

3.合并子区间

void merge_sort(int q[], int l, int r)
{
   	if (l >= r) return;

    int mid = l + r >> 1;
    merge_sort(q, l, mid);
    merge_sort(q, mid + 1, r);

    int k = 0, i = l, j = mid + 1;
    while (i <= mid && j <= r)
        if (q[i] <= q[j]) tmp[k ++ ] = q[i ++ ];
        else tmp[k ++ ] = q[j ++ ];

    while (i <= mid) tmp[k ++ ] = q[i ++ ];
    while (j <= r) tmp[k ++ ] = q[j ++ ];
    for (i = l, j = 0; i <= r; i ++, j ++ ) q[i] = tmp[j];
}