# Interview Questions

# Compute x^n (x to the power n)

- September 25, 2013
- Posted by: Kamal Rawat
- Category: Algorithms

Write a C function to compute x^{n} which uses minimum number of multiplications.

unsigned int power(double x, unsigned int n)

**Brute Force Solution: O(n) time**

The brute force method for this is to use a for loop

double power(double x, unsigned int n) { double product = 1; for(int i=0; i<n; i++) { product = product * x; } return product; }

We can also write the function recursively. But recursion gives no advantage in terms of performance. In fact it is always less optimal than non-recursive version.

double power(double x, unsigned int n) { if (n == 0) return 1; return x * power(x, n-1); }

**Time Complexity:** O(n)

**Space Complexity:** For non-recursive version its O(1), for recursive version its O(n).

**Optimized Solution: O(lg(n)) time**

A Better Solution is to do Exponentiation by squaring. In this method we take advantage of the basic formula:

x^{n+m} = x^{m} * x^{n}

Hence,

x^{n} = x^{n/2} * x^{n/2 } (If x is even)

x^{n} = x * x^{n/2} * x^{n/2} (If x is odd)

double power(double x, unsigned int n) { if( n == 0) return 1; double retValue = 1; while(n) { if(n%2) retValue = retValue * x; x = x * x; n = n/2; } return retValue; }

The recursive version of above code will be something like:

double power(double x, unsigned int n) { if( n == 0) return 1; double temp = power(x, n/2); if (n%2 == 0) return temp*temp; else return x*temp*temp; }

*Note that the above code is not same as the below code* (though both are recursive and both does exponential multiplication)

unsigned int power(double x, unsigned int n) { if (n == 0) return 1; if(x%2 == 0) return power(x, n/2) * power(x, n/2); else return x * power(x, n/2) * power(x, n/2); }

In the above code we are calling power method twice and hence it will be much more time consuming. (if n=10) then power(x,5) will be called twice. Its a typical problem with recursion.

The approach in the first recursive function is that call power(x,5) once and store its value in a variable (temp) and for second time use that value. This approach is called Dynamic Programming.

You can learn more about Dynamic Programming here, here or here…

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