Desafío de programación Factorial

Problema

The factorial of n, written n!, is 1 × 2 × … × n. By definition 0! = 1.

Implement factorial(n).

n: A non-negative integer.

Return n!.

0 ≤ n ≤ 20, so the result fits a signed 64-bit integer.

Ejemplo 1

Entrada:
n = 5

Salida esperada:
expected = 120

Entrada:
long long n = 5;

Salida esperada:
long long expected = 120;

Entrada:
long n = 5;

Salida esperada:
long expected = 120;

1 × 2 × 3 × 4 × 5 = 120.

Ejemplo 2

Entrada:
n = 0

Salida esperada:
expected = 1

Entrada:
long long n = 0;

Salida esperada:
long long expected = 1;

Entrada:
long n = 0;

Salida esperada:
long expected = 1;

0! is defined as 1.