详细推导:https://leetcode-cn.com/problems/jian-sheng-zi-lcof/solution/mian-shi-ti-14-i-jian-sheng-zi-tan-xin-si-xiang-by/

class Solution {
public:
    int cuttingRope(int n) {
        if (n <= 3) return 1 * (n - 1);
        int res = 1;
        if (n % 3 == 1) res *= 4,n -= 4;
        if (n % 3 == 2) res *= 2,n -= 2;
        while (n)  res *= 3,n -= 3;
        return res;
    }
};