二維前綴和

    for (int i = 1; i <= n; i++)
        for (int j = 1; j <= m; j++) {
            h[i][j] = Read();
            sum[i][j] = sum[i - 1][j] + sum[i][j - 1] - sum[i - 1][j - 1] + h[i][j];
        }//二維前綴和