我开始想用map.lower_bound的，但是报错了，就直接暴力搜索了。

class Solution {
public:
    string intToRoman(int num) {
        map<int,string> mp;
        mp[1] = "I";
        mp[4] = "IV";
        mp[5] = "V";
        mp[9] = "IX";
        mp[10] = "X";
        mp[40] = "XL";
        mp[50] = "L";
        mp[90] = "XC";
        mp[100] = "C";
        mp[400] = "CD";
        mp[500] = "D";
        mp[900] = "CM";
        mp[1000] = "M";
        string ans = "";

            while(num != 0)
            {
                //map<int,char>::iterator it = mp.lower_bound(num,greater<int>());
                map<int,string>::iterator it;
                for(map<int,string>::iterator iter = mp.begin();iter != mp.end();iter++)
                {
                    if(iter->first > num)
                    {
                        break;
                    }
                    it = iter;
                }
                num -= it->first;
                ans += it->second;
            }
        return ans;
    }
};