Evaluate Reverse Polish Notation

class Solution
{
public:
    int evalRPN(vector<string> &tokens)
    {
       // if(tokens.size()==1) return tokens[0];
        stack<int>S;
        for(int i=0; i<tokens.size(); i++)
        {
            if(!(tokens[i][0]>=0 && tokens[i][0]<=9))
            {
                if((tokens[i][0]>='0' && tokens[i][0]<='9'))
                {
                    int n;
                    sscanf(tokens[i],"%d",&n);
                    S.push(n);
                }
                else if(!(tokens[i][0]>='0' && tokens[i][0]<='9') && (tokens[i][1]>='0' && tokens[i][1]<='9'))
                {
                    int n;
                    sscanf(tokens[i],"%d",&n);
                    S.push(n);
                }
                else
                {
                    int n,m;
                    m=S.top();S.pop();
                    n=S.top();S.pop();
                    switch(tokens[i][0])
                    {
                    case '+':S.push(m+n);break;
                    case '-':S.push(n-m);break;
                    case '*':S.push(n*m);break;
                    case '/':S.push(n/m);break;
                    }
                }
            }
        }
        return S.top();
    }
};