leetcode - [2]Evaluate Reverse Polish Notation

#include <iostream>
#include <vector>
#include <string>
#include <stack>
#include <cstdlib>

using namespace std;


class Solution {
public:
    int evalRPN(vector<string> &tokens) {
        stack<int> s;
        string str;
        int a, b;

        for (int i = 0; i < tokens.size(); i++) {
            str = tokens[i];
            if (str == "+" || str == "-" || str == "*" || 
                a = s.top(); s.pop();
                b = s.top(); s.pop();
                switch(str[0]) {
                    case ‘+‘: s.push(b+a); break;
                    case ‘-‘: s.push(b-a); break;
                    case ‘*‘: s.push(b*a); break;
                    case ‘/‘: s.push(b/a); break;
                    }
            }
            else {
                s.push(atoi(str.c_str()));
            }
        }
        a = s.top();s.pop();
        return a;
    }
};


int main(int argc, char *argv[]) {
    Solution* solution = new Solution();
    vector<string> tokens;
    tokens.push_back("2");
    tokens.push_back("1");
    tokens.push_back("+");
    tokens.push_back("3");
    tokens.push_back("*");

    cout << solution->evalRPN(tokens) << endl;

    vector<string> tokens2;
    tokens2.push_back("4");
    tokens2.push_back("13");
    tokens2.push_back("5");
    tokens2.push_back("/");
    tokens2.push_back("+");
    cout << solution->evalRPN(tokens2) << endl;
    return 0;
}