数学O（1）法

class Solution {
public:
    int cutRope(int number) {
        int tmp = number % 3;
        if(number < 4) return number-1;
        switch(tmp){
            case 0:
                return pow(3,number/3);
            case 1:
                return 4*pow(3,(number-4)/3);
            case 2:
                return 2*pow(3,number/3);
        }
    }
};