剑指 Offer 16. 数值的整数次方

    private int[] arrTwo;
    private Map<Integer,Double> arrRes=new HashMap<>();

    public double myPow(double x, int n) {
        if(n==0){
            return 1;
        }
        arrTwo=new int[32];
        arrTwo[0]=1;
        arrTwo[1]=2;
        for (int i = 2; i < 32; i++) {
            arrTwo[i]=arrTwo[i-1]*2;
        }
        if (n<0){
            x=1/x;
            n=-n;
        }
        int i=2;
        arrRes.put(1,x);
        return func(x,n);
    }

    public double func(double x,int n){
        if (arrRes.containsKey(n)){
            return arrRes.get(n);
        }
        double res=x;
        int i=2;
        for ( ; n-arrTwo[i-1]>=0; i++) {
            res*=res;
            arrRes.put(arrTwo[i-1],res);
            if(n==arrTwo[i-1]){
                return res;
            }
        }
        res*=func(x,n-arrTwo[i-2]);
        return res;
    }