Implement pow(x, n).
class Solution {
func power(var x: Double, var _ n: Int) -> Double {
if x == 0 {return 0}
if n == 0 {return 1}
if n < 0 {
n = -n
x = 1/x
}
return (n%2 == 0) ? power(x*x, n/2) : power(x*x, n/2) * x
}
}
