Solutions to lightbox entry question

lightbox.c

// This one will probably take up linear stack size in most compilers although it shouldn't
f(int n,int*a){return n-1?n-*a+f(n-1,a+1):1;}

lightbox.java

// A proper solution
static int find_missing_number(int n, int[] numbers)
{
	long sum = (long)n * (n + 1) / 2;
	for (int x : numbers)
		sum -= x;
	return (int)sum;
}

lightbox.py

def find_missing_number(n, numbers):
  return n * (n + 1) / 2 - sum(numbers)