Leetcode OJ : Evaluate Reverse Polish Notation Stack C++ solution

 #define ADDITION  '+'
 #define SUBSTRACTION  '-'
 #define MULTIPLICATION  '*'
 #define DIVISION  '/'

 class Solution {
 public:
     set<char> tokenSet{'+', '-', '*', '/'};
     stack<int> num;
     int evalRPN(vector<string> &tokens) {
         for (auto &token : tokens) {
             auto iter = tokenSet.find( token[token.size()-] );
             if ( iter == tokenSet.end() ) {
                 num.push( atoi( token.c_str() ) );
             } else {
                int right = num.top();
                num.pop();
                int left = num.top();
                num.pop();
                switch ( *iter )
                {
                    case ADDITION :
                        num.push(left + right);
                        break;
                    case SUBSTRACTION:
                        num.push(left - right);
                        break;
                    case MULTIPLICATION:
                        num.push(left * right);
                        break;
                    case DIVISION:
                        num.push(left / right);
                        break;
                    default:
                        break;
                }
             }
         }
         int ret = num.top();
         num.pop();
         return ret;
     }
 };