Evaluate Reverse Polish Notation Solutions in C++

// Source : https://oj.leetcode.com/problems/evaluate-reverse-polish-notation/
// Author : Hao Chen
// Date   : 2014-06-16



#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string>
#include <vector>
#include <iostream>
using namespace std;


class Solution {
public:
    int evalRPN(vector<string> &tokens) {
        int i =0;
        bool err = false;
        vector<int> exp;
        for (int i=0; i<tokens.size() && !err; i++ ){
            if (isNum(tokens[i])) {
                exp.push_back(value);
            } else if( isOp(tokens[i])==true ) {
                if (exp.size() < 2) {
                    return 0; //ERROR
                }
                int lhs, rhs;
                rhs = exp.back();
                exp.pop_back();
                lhs = exp.back();
                exp.pop_back();

                int evlValue;
                if (tokens[i]=="+"){
                    evlValue = lhs + rhs;    
                }else if (tokens[i]=="-"){
                    evlValue = lhs - rhs;
                }else if (tokens[i]=="*"){
                    evlValue = lhs * rhs;
                }else if (tokens[i]=="/"){
                    evlValue = lhs / rhs;
                }

                exp.push_back(evlValue);

            }else {
                return 0; //ERROR 
            }
        }

        if (exp.size()==1){
            return exp.back();
        }
        return 0;        

    }

private:
    long value;

    bool isOp(string &op) {
        return (op=="+" || op=="-" || op=="*" || op=="/");
    }

    bool isNum(string &num) {
        char *end;
        value = strtol(num.c_str(), &end, 10); 
        if (end == num.c_str() || *end != '\0' || errno == ERANGE){
            return false;
        }
        return true;
    }
};


int main()
{
    Solution s;
    char exps[5][3] = {"42", "9", "6", "-", "+"};
    vector<string> expression;

    cout << "Expression: \n    ";
    for (int i=0; i<5; i++){
        expression.push_back(exps[i]);
        cout << exps[i] << " ";
    }
    cout << endl;
    cout << s.evalRPN(expression)<<endl;;

    char exps2[5][3] =  {"2", "1", "+", "3", "*"};
    expression.clear();

    cout << "Expression: \n    ";
    for (int i=0; i<5; i++){
        expression.push_back(exps2[i]);
        cout << exps2[i] << " ";
    }
    cout << endl;
    cout << s.evalRPN(expression)<<endl;



    return 0;
}

/// Source : https://leetcode.com/problems/evaluate-reverse-polish-notation/description/
/// Author : liuyubobobo
/// Time   : 2018-08-29
/// Updated: 2019-08-05

#include <iostream>
#include <stack>
#include <vector>

using namespace std;


/// Using Stacks
/// Time Complexity: O(n)
/// Space Complexity: O(n)
class Solution {
public:
    int evalRPN(vector<string>& tokens) {

        stack<int> nums;
        for(const string& s: tokens){
            if(s == "+" || s == "-" || s == "*" || s == "/"){
                int a = nums.top();
                nums.pop();
                int b = nums.top();
                nums.pop();

                if(s == "+")
                    nums.push(b + a);
                else if(s == "-")
                    nums.push(b - a);
                else if(s == "*")
                    nums.push(b * a);
                else if(s == "/")
                    nums.push(b / a);
            }
            else
                nums.push(atoi(s.c_str()));
        }
        return nums.top();
    }
};


int main() {

    return 0;
}