Factorial Algorithm

The factorial of a number is the product of all positive integers less than or equal to it. e.g. the factorial of 5 is 5 x 4 x 3 x 2 x 1 = 120.

While conducting a Java interview, I like to ask my candidates to write an algorithm to calculate the factorial of a number. This is one of my favourite questions because it gives me an insight into their thinking process. It is also one of the questions that I was asked at Merrill Lynch, in my first interview out of university. (I got it right)

I am surprised at how the majority of people cannot answer this question, or get anywhere close to it. C'mon, you can do it using recursion or loop-iteration; its not that difficult! If you can't count backwards, do it forwards - its the same thing.

Here is a simple solution: //loop public int factorial(int n) { int result = 1 ; for(int i = n ; i > 0 ; i--) { result = result * i ; } return result ; } //recursion public int recursiveFactorial(int n) { if (n == 0) return 1 ; else return n * recursiveFactorial(n-1) ; }