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

Solution: stack.

1 class Solution { 2 public: 3     int evalRPN(vector
      
        &tokens) { 4         stack
       
         s; 5         for (int i = 0; i < tokens.size(); ++i) { 6             if (tokens[i] != "+" && tokens[i] != "-" && tokens[i] != "*" && tokens[i] != "/") 7                 s.push(stoi(tokens[i])); 8             else { 9                 int op2 = s.top();s.pop();10                 int op1 = s.top();s.pop();11                 int result = 0;12                 if(tokens[i] == "+")13                     result = op1 + op2;14                 else if(tokens[i] == "-")15                     result = op1 - op2;16                 else if(tokens[i] == "*")17                     result = op1 * op2;18                 else if(tokens[i] == "/")19                     result = op1 / op2;20                 s.push(result);21             }22         }23         return s.top();24     }25 };