Function combination

This method calculates the number of unordered subsets of k elements taken from a set of n elements, also known as 'n choose k'.

Remarks

• The method uses the formula C(n,k) = n!/(k!(n-k)!) to calculate the number of combinations.
• Requires n > k >= 0 to work properly. Otherwise, if k >= n, the method returns the value of n!/k!