Help the bookseller !（C++CodeWars）

解題思路：

（1）使用map

using std::vector;
using std::string;
using std::cout;
using std::endl;
using std::unordered_map;
using std::stringstream;
using std::to_string;


class StockList  {
public:
  static string stockSummary(vector<string> &lstOfArt, vector<string> &categories) {
    if(lstOfArt.size()==0 || categories.size()==0) return "";
    unordered_map<char,int> mp;
    string strc,strn;
    for(auto&& w:lstOfArt) {
      stringstream s(w);
      s>>strc;s>>strn;
      mp[strc[0]]+=atoi(strn.c_str());
    }
    strc = "";
    for(auto&& w:categories) {
      strc = strc+"("+w+" : "+to_string(mp[w[0]])+") - ";
    }
    return strc.substr(0,strc.length()-3);
  }
};