LeetCode() Largest Number

class Solution {
public:
    string largestNumber(vector<int>& nums) {
        sort(nums.begin(),nums.end());
        string res="";
        res=f(nums);
        while(next_permutation(nums.begin(),nums.end())){
            if(res<f(nums))
                res=f(nums);
        }
        return res;
    }
    string f(vector<int>& coll){
        string res;
	for(int i=0;i<coll.size();++i){
		ostringstream sss;
		sss<<coll[i];
		res+=sss.str();
	}
	return res;
    }
};

class Solution {
public:
    string largestNumber(vector<int>& nums) {
        vector<string> coll;
        for(int i=0;i<nums.size();++i){
            ostringstream sss;
		    sss<<nums[i];
		    coll.push_back(sss.str());
        }
        sort(coll.begin(),coll.end());
        string res="";
        for(int i=coll.size()-1;i>=0;--i)
            res+=coll[i];
        return res;
    }
    
};