剑指offer-把数组排成最小的数

bool Cmp(int a, int b){
    int a_num = 0, b_num = 0, at = a, bt = b;
    while(at){
        a_num++; 
        at /= 10; 
    }
    while(bt){
        b_num++; 
        bt /= 10; 
    }
    at = b*pow(10, a_num) + a; 
    bt = a*pow(10, b_num) + b; 
    return bt < at; 
}

class Solution {
public:
    string PrintMinNumber(vector<int> numbers) {
        sort(numbers.begin(), numbers.end(), Cmp); 
        string ans = ""; 
        for(int i=0; i<numbers.size(); ++i){
            ans += to_string(numbers[i]); 
        }
        return ans; 
    }
};