#include <cassert>
#include <functional>
#include <string>
#include <vector>

class Solution {
    static auto get_function(const std::string &op)
        -> std::function<int(int, int)> {
        if (op == "+") {
            return std::plus<int>{};
        } else if (op == "-") {
            return std::minus<int>{};
        } else if (op == "*") {
            return std::multiplies<int>{};
        } else if (op == "/") {
            return std::divides<int>{};
        }

        assert(false);
    }

  public:
    auto evalRPN(const std::vector<std::string> &tokens) -> int {
        std::vector<int> stack;

        for (const auto &token : tokens) {
            if (token == "+" || token == "-" || token == "*" || token == "/") {
                assert(stack.size() >= 2);

                int right = stack.back();
                stack.pop_back();
                int left = stack.back();
                stack.pop_back();

                stack.push_back(get_function(token)(left, right));
            } else {
                stack.push_back(std::stoi(token));
            }
        }

        return stack.back();
    }
};

int main() {
    Solution s;

    assert(s.evalRPN(std::vector<std::string>{
               {"2"}, {"1"}, {"+"}, {"3"}, {"*"}}) == 9);
    assert(s.evalRPN(std::vector<std::string>{
               {"4"}, {"13"}, {"5"}, {"/"}, {"+"}}) == 6);
    assert(s.evalRPN(std::vector<std::string>{{"10", "6", "9", "3", "+", "-11",
                                               "*", "/", "*", "17", "+", "5",
                                               "+"}}) == 22);

    return 0;
}