Implement pow(x, n).

pow.py

class Solution:
    # @param x, a float
    # @param n, a integer
    # @return a float
    def pow(self, x, n):
        if x == 0.0:
            return 0.0
        if n < 0:
            # Avoids arithmetic underflow
            return 1.0/self.pow(x, -n)
  
        if n == 0:
            return 1.0
        elif n % 2 == 0:
            return self.pow(x, n // 2) ** 2
        else:
            return x * self.pow(x, n // 2) ** 2