Calculate the factorial of a non-negative integer. The factorial of n (written as n!) is the product of all positive integers less than or equal to n. By definition, 0! = 1 and 1! = 1. Return null for negative numbers and null input.

Examples:

Input: n = 5 Output: 120 Explanation: 5! = 5 x 4 x 3 x 2 x 1 = 120 Input: n = 0 Output: 1 Explanation: By definition, 0! = 1 Input: n = 1 Output: 1 Explanation: By definition, 1! = 1 Input: n = 10 Output: 3628800 Explanation: 10! = 10 x 9 x 8 x 7 x 6 x 5 x 4 x 3 x 2 x 1 = 3628800 Input: n = -1 Output: null Explanation: Negative numbers do not have factorials Input: n = null Output: null Explanation: Null input returns null
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