Factorial

$n! = 1 \cdot 2 \cdot \ldots \cdot n$, $0! = 1$. Compute $n!$ with a recursive function: $0! = 1$ is the base case, $n! = n \cdot (n - 1)!$ moves to a smaller problem.

A single integer $n$ ($0 \le n \le 20$).

A single integer — $n!$.

A recursive function has two parts: the base case (returns an answer without calling itself) and the recursive step (calls itself with a smaller argument). Without a base case the function calls itself forever.

$20! = 2\,432\,902\,008\,176\,640\,000$ does not fit in int — use a 64-bit type (long long, int64).