# leetle

## 21. Factorial

Write a function `solve` that returns the factorial of a given integer n. Return 1 if n is 0. The factorial of a non-negative integer n is the product of all positive integers less than or equal to n.

**Example:** `Input: 5` `Output: 120`

Make sure you `return` your solution, don't print!

```python

def solve(n):
    pass
```