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

class Solution {
public:
    int evalRPN(vector<string> &tokens) 
    {
        stack<int> vals;
        if(tokens.size()==0) return 0;
        
        for(int i=0;i<tokens.size();i++)
        {
            string token=tokens[i];
            if(token=="+" || token=="-" || token=="*" || token=="/")
            {
                int y=vals.top();vals.pop();
                int x=vals.top();vals.pop();
                if(token=="+") vals.push(x+y);
                if(token=="-") vals.push(x-y);
                if(token=="*") vals.push(x*y);
                if(token=="/") vals.push(x/y);
            }
            else
            {
                vals.push(string2int(token));
            }
        }
        return vals.top();
    }
    int string2int(string s)
    {
        bool positive=true;
        if(s[0]=='-') positive=false;
        
        int result=0;
        for(int i=0;i<s.length();i++)
        if(s[i]!='-')
         result=result*10+s[i]-'0';
        return positive?result:-result;
    }
};