Interview Question for Software Engineering Intern at Facebook:

Could be implemented many ways. I got the feeling that the interviewer wanted to see you approach the problem in multiple ways and demonstrate confidence in your math and recursive skills.

#include <stdlib.h>
#include <stdio.h>

#define MAX_ARRAY_LENGTH 256

double power(double, unsigned int);

int main(int argc, char** argv) {
    double a = atof(argv[1]);
    int b = atoi(argv[2]);
    double result = power(a, b >> 31 == 0 ? b : -b);
    if ((unsigned int) b >> 31 == 1) {
        result = 1 / result;
    }
    printf("%f\n", result);
    return 0;
}

double power(double a, unsigned int b) {
    switch (b) {
    case 0:
        return 1.0;
    case 1:
        return a;
    default:
        return (b & 1) == 0 ? power(a * a, b >> 1) : power(a * a, b >> 1) * a;
    }
}

c implementation of the above (no recursion):
int ipow(int base, int exp){
    int result = 1;
    while(exp){
        if(exp & 1) { result *= exp; }
        exp >>= 1;
        base *= base;
    }
    return result;
}

int power(double n, int exp)
{
    bool npower = (exp < 0) ? true : false;
    double result = 1;

    exp = abs(exp); // get the absolute value

    for (int i = 0; i < exp; i++)
    {
        if (npower)
        {
            result = result/n;
        }
        else
        {
            result = result*n;
        }
     }

    return result;
}

C# code verified:

        static double Power(double d, int exp)
        {
            if (d == 0 || exp == 0)
            {
                if (exp >= 0)
                {
                    return 1;
                }
                else
                {
                    return double.PositiveInfinity;
                }
            }

            int expAbs = Math.Abs(exp);
            double res = d;
            for (int i = 1; i < expAbs; i++)
            {
                res *= d;
            }

            return (exp > 0) ? (res) : (1 / res);
        }