LeetCode 50. Pow(x, n)

边界检查：0，INT_MAX, INT_MIN

class Solution {
private:
    map<int, double> memory;
public:
    double myPow(double x, int n) {
        if(n == 0)
            return 1.0;
        if(n == 1)
            return x;
        if(n == -1)
            return 1/x;
        if(memory.count(n)>0)
            return memory[n];
        double result = myPow(x, n/2) * myPow(x, n/2);
        if(n%2 != 0){
            result *= (n<0)? 1/x : x;
        }
        memory[n] = result;
        return result;
    }
};