`class Solution {
  public:
    string postToInfix(string exp) {
        // Write your code here
        stack<string>st;
        for(int i=0;i<exp.length();i++){
            if(!isalpha(exp[i])){
                string A=st.top();  st.pop();
                string B=st.top();  st.pop();
                
                string s="(" + B + exp[i] + A + ")";
                st.push(s);
            }
            else st.push(string(1,exp[i]));
        }
        return st.top();
    }
};`

• The problem is from geeks of geeks.I am getting time limit exceeded.What can I do to optimize it? problem link