150. Evaluate Reverse Polish Notation (Stack)

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

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

Some examples:

  ["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9
  ["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6

class Solution {
public:
    int evalRPN(vector< string > &tokens) {
        stack< int > operandStack;
        int operand1;
        int operand2;
        for(int i = ; i < tokens.size(); i++){
            if(tokens[i]=="+"){
                operand1 = operandStack.top();
                operandStack.pop();
                operand2 = operandStack.top();
                operandStack.pop();
                operand2 += operand1;
                operandStack.push(operand2);
            }
            else if(tokens[i]=="-"){
                operand1 = operandStack.top();
                operandStack.pop();
                operand2 = operandStack.top();
                operandStack.pop();
                operand2 -= operand1;
                operandStack.push(operand2);
            }
            else if(tokens[i]=="*"){
                operand1 = operandStack.top();
                operandStack.pop();
                operand2 = operandStack.top();
                operandStack.pop();
                operand2 *= operand1;
                operandStack.push(operand2);
            }
            else if(tokens[i]=="/"){
                operand1 = operandStack.top();
                operandStack.pop();
                operand2 = operandStack.top();
                operandStack.pop();
                operand2 /= operand1;
                operandStack.push(operand2);
            }
            else{
                operand1 = atoi(tokens[i].c_str());
                operandStack.push(operand1);
            }
        }
        return operandStack.top();
    }
};