# Number With Exclamation

Elementary+
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This function should take a non-negative integer as an input and return the factorial of that number. The factorial of a non-negative integer n is the product of all positive integers less than or equal to n .

Input: Integer (int).

Output: Integer (int).

Examples:

```assert factorial(0) == 1
assert factorial(1) == 1
assert factorial(5) == 120
assert factorial(10) == 3628800
```

How it’s used:

• in mathematical applications to calculate permutations and combinations;
• in algorithms to solve problems related to counting arrangements.

Precondition:

• n ∈ N₀

Settings
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