leetcode Evaluate Reverse Polish Notation(计算逆波兰表达式)

题目要求：

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

我的c++代码:

int evalRPN(vector<string> &tokens) 
    {
      if(tokens.size() == 0)
        return 0;
      string op = "+-*/";
      stack<int> st;
      for(size_t i = 0; i < tokens.size(); ++i)
      {
        int flag = op.find(tokens[i]);
        if(flag != string::npos)
        {
          if(st.size() < 2)
            return -1;
          int a = st.top();
          st.pop();
          int b = st.top();
          st.pop();
          if(flag == 0)
            st.push(a + b);
          if(flag == 1)
            st.push(b - a);
          if(flag == 2)
            st.push(a * b);
          if(flag == 3)
            st.push(b / a);  
        }
        else 
          st.push(atoi(tokens[i].c_str()));
      }
      return st.top();
    }