150. Evaluate Reverse Polish Notation

Evaluate the value of an arithmetic expression in Reverse Polish Notation.

Valid operators are +, -, *, /. Each operand may be an integer or another expression.

Note:

• Division between two integers should truncate toward zero.
• The given RPN expression is always valid. That means the expression would always evaluate to a result and there won't be any divide by zero operation.

Example 1:

Input: ["2", "1", "+", "3", "*"]
Output: 9
Explanation: ((2 + 1) * 3) = 9

Example 2:

Input: ["4", "13", "5", "/", "+"]
Output: 6
Explanation: (4 + (13 / 5)) = 6

Example 3:

Input: ["10", "6", "9", "3", "+", "-11", "*", "/", "*", "17", "+", "5", "+"]
Output: 22
Explanation: 
  ((10 * (6 / ((9 + 3) * -11))) + 17) + 5
= ((10 * (6 / (12 * -11))) + 17) + 5
= ((10 * (6 / -132)) + 17) + 5
= ((10 * 0) + 17) + 5
= (0 + 17) + 5
= 17 + 5
= 22

class Solution {
private:
    set<string> operators = {"+","-","*","/"};
    stack<int> sk_rpn;
public:
    int evalRPN(vector<string>& tokens) {
        if(!tokens.size())
            return 0;
        for(int i=0; i<tokens.size(); i++)
        {
            if(operators.find(tokens[i])==operators.end())
            {
                sk_rpn.push(atoi(tokens[i].c_str()));
            }else
            {
                    if(tokens[i].compare("+")==0)
                    {
                        int num2 = sk_rpn.top();
                        sk_rpn.pop();
                        int num1 = sk_rpn.top();
                        sk_rpn.pop();
                        int result = num1 + num2;
                        sk_rpn.push(result);
                    }else if(tokens[i].compare("-")==0)
                    {
                        int num2 = sk_rpn.top();
                        sk_rpn.pop();
                        int num1 = sk_rpn.top();
                        sk_rpn.pop();
                        int result = num1 - num2;
                        sk_rpn.push(result);
                    }else if(tokens[i].compare("*")==0)
                    {
                        int num2 = sk_rpn.top();
                        sk_rpn.pop();
                        int num1 = sk_rpn.top();
                        sk_rpn.pop();
                        int result = num1 * num2;
                        sk_rpn.push(result);
                    }else if(tokens[i].compare("/")==0)
                    {
                        int num2 = sk_rpn.top();
                        sk_rpn.pop();
                        int num1 = sk_rpn.top();
                        sk_rpn.pop();
                        int result = num1 / num2;
                        sk_rpn.push(result);
                    }
                
            }
        }
        int result = sk_rpn.top();
        sk_rpn.pop();
        return result;
    }
};