LeetCode 179 Largest Number 把数组排成最大的数

Given a list of non negative integers, arrange them such that they form the largest number.
For example, given [3, 30, 34, 5, 9], the largest formed number is 9534330.
Note: The result may be very large, so you need to return a string instead of an integer.

 class Solution {
 public:
     string largestNumber(vector<int>& nums) {
         vector<string> arr;
         for(int n:nums)
             arr.push_back(to_string(n));
         sort(arr.begin(),arr.end(),[](string &str1,string &str2){return str1+str2>str2+str1;});
         string res;
         for(string s:arr)
             res+=s;
         if(res[]=='')
             return "";
         return res;
     }
 };