Tail v. Head Recursion

Question

I was wondering if you would identify this as a head or tail recursive function:

int exponentiation(int x, int y){
    if(!y) { return 1; }
    return y > 1 ? x * exponentiation(x, y-1) : x;
}

Answer 1

This is not a tail recursion: returning the result of exponentiation is not the last action taken by the function; the multiplication by x is.

However, the function is easy to convert to a tail-recursive implementation by adding an extra parameter, exp:

int exponentiation_tail(int x, int y, int exp = 1){
    if(y <= 0) { return exp; }
    return exponentiation_tail(x, y-1, exp*x);
}