function uniquePaths(m: number, n: number): number {
    const f: number[] = Array(n).fill(1);
    for (let i = 1; i < m; ++i) {
        for (let j = 1; j < n; ++j) {
            f[j] += f[j - 1];
        }
    }
    return f[n - 1];
}