https://leetcode-cn.com/problems/powx-n/
class Solution {
public double erfen(double x, long N) {
if (N == 0) {
return 1.0;
}
double y = erfen(x, N / 2);
return N % 2 == 0 ? y * y : y * y * x;
}
public double myPow(double x, int n) {
long N = n;
return N >= 0 ? erfen(x, N) : 1.0 / erfen(x, -N);
}
}