ll res = -oo;
    for(int k=1; k<=n; k++)
        for(int i=1; i<=n-k+1; i++)
            for(int j=1; j<=n-k+1; j++)
            {
                f[i][j][k] = f[i][j][k - 1] + a[i + k - 1] * b[j + k - 1];
                res = max(res, f[i][j][k]);
            }
    cout << res;

ll res = -oo;
    for(int i=1; i<=n; i++)
        for(int j=1; j<=n; j++)
            for(int k=1; k<=n; k++)
            {
                if(k + i > n || k + j > n) break;
                f[i][j][k] = f[i][j][k - 1] + a[i + k - 1] * b[j + k - 1];
                res = max(res, f[i][j][k]);
            }
    cout << res;

These code seem not to be differ too much but execution time have much differ. Example in case n = 500, code1 run in 3103ms but code 2 run only 789ms. What is the reason ? Pls explain to me. Thanks.