LeetCode: 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.

Some examples:

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

   
   bool number(string &s)  
   {  
     char ch = s[0];  
     if(ch == '-')ch = s[1];  
     if (ch >= '0' && ch <= '9')return true;  
     return false;  
   }



   int evalRPN(vector<string> &tokens) {  
     stack<int> s;  
     int res = 0;  
     for (decltype(tokens.begin()) it = tokens.begin(); it != tokens.end(); it++)  
     {  
       if (number(*it))  
       {  
         s.push(stoi(*it));  
       }  
       else  
       {  
         string op = *it;  
         int a = s.top();  
         s.pop();  
         int b = s.top();  
         s.pop();  
         switch(op[0])  
         {  
           case '+':  
             res = a+b;  
             break;  
           case '-':  
             res = b-a;  
             break;  
           case '*':  
             res = a*b;  
             break;  
           case '/':  
             res = b/a;  
             break;  
         }  
         s.push(res);  
       }  
     }  
     return s.top();  
   }