150. Evaluate Reverse Polish Notation

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

​​+​​, ​​-​​, ​​*​​, ​​/​​. 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

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class Solution {
public:
  int evalRPN(vector<string>& tokens) {
    stack<int> stk;
    for (string str : tokens){
      if (!isOp(str)){
        stk.push(stoi(str));
      }
      else{
        int num1 = stk.top(); stk.pop();
        int num2 = stk.top(); stk.pop();
        int num=0;
        switch (str[0])
        {
          case '+':num = num1 + num2; break;
          case '-':num = num2 - num1; break;
          case '*':num = num1*num2; break;
          case '/':num = num2 / num1; break;
        default:
          break;
        }
        stk.push(num);
      }
    }
    return stk.top();
  }
private:
  bool isOp(string str){
    if (str.size()==1&&(str[0] == '+' || str[0] == '-' ||
      str[0] == '*' || str[0] == '/')){
      return true;
    }
    return false;
  }
};