Given a positive integer, N. Find the factorial of N.

Example 1:
Input:
N = 5
Output:
120
Explanation:
54321 = 120
Example 2:
Input:
N = 4
Output:
24
Explanation:
432*1 = 24
Your Task:
You don't need to read input or print anything. Your task is to complete the function factorial() which takes an integer N as input parameters and returns an integer, the factorial of N.

Expected Time Complexity: O(N)
Expected Space Complexity: O(1)

Constraints:
0 <= N <= 18

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