解题思路：

由于数量级较大，需要使用long以及快速幂求余

下图便于理解快速幂求余

class Solution {
    public int cuttingBamboo(int bamboo_len) {
        if(bamboo_len <= 3) return bamboo_len - 1;
        int b = bamboo_len % 3, p = 1000000007;
        long rem = 1, x = 3;
        for(int a = bamboo_len / 3 - 1; a > 0; a /= 2) {//a减一是为了少切分一个3，留给后面b的取值讨论
            if(a % 2 == 1) rem = (rem * x) % p;
            x = (x * x) % p;
        }
        if(b == 0) return (int)(rem * 3 % p);
        if(b == 1) return (int)(rem * 4 % p);
        return (int)(rem * 3 * 2 % p);
    }
}