把数组排成最小的树

class Solution {
public:
    string PrintMinNumber(vector<int> numbers) {
        string res;
        if(numbers.size()==0) return res;
        sort(numbers.begin(),numbers.end(),cmp);
        for(int i=0;i<numbers.size();i++){
            res+=to_string(numbers[i]);
        }
        return res;
    }
    static bool cmp(int a,int b){
        string str1 = to_string(a);
        string str2 = to_string(b);
        return str1+str2 < str2+str1;
    }
};