These algorithms are mostly from Standish, T. (1995),
Data Structures, Algorithms & Software Principles in C.

Power1 - contains a recursive method that computes x^n, called power(x, n),
         where x is a floating point number and n is a nonnegative integer.
         The algorithm is naturally recursive, and O(n).
           Base cases:
             power(x, 0) = 1.0
             power(x, 1) = x
           General case:
             power(x, n) = x * power(x, n-1), for n > 1

Power2 - an improved recursive version of power(x, n) works by breaking n
         down into halves (where half of n = n / 2), squaring power(x, n / 2),
         and multiplying by x again if n was odd. For example,
         x^11 = (x^5) * (x^5) * x, whereas x^10 = (x^5) * (x^5).
         This algorithm is O(log n).

Here are some sample runs, using our Power2 solution:

% java Power1 1.2 50
1.2 to the power of 50 is 9100.438150002134
    (power called 50 times)

% java Power2 1.2 50
1.2 to the power of 50 is 9100.438150002139
    (power called 6 times)

% java Power2 -3 7
-3.0 to the power of 7 is -2187.0
    (power called 3 times)

% java Power1
2 arguments required

% java Power2 1.2
2 arguments required

% java Power1 3 -2
power must be positive