Code that converts infix into postfix

infixToPost.cpp

#include <bits/stdc++.h>
using namespace std;

int precedence(char k){
  if(k == '^') return 3;
  else if(k == '/' || k =='*') return 2;
  else if(k == '+' || k == '-') return 1;
  else return -1;
}

int main() {
  stack<char> st;
  string s;
  cin>>s;
  for(auto i:s){
    if((i>='a' && i<='z')||(i>='A' && i<='Z')||(i>='0' && i<='9')){
      cout<<i;
    }
    else if(i=='(') st.push(i);
    else if(i==')'){
      while(st.top()!='('){
        cout<<st.top();
        st.pop();
      }
      st.pop();
    }
    else{
      while(!st.empty()&& precedence(i)<=precedence(st.top())){
        cout<<st.top();
        st.pop();
      }
      st.push(i);
    }
  }
  while(!st.empty()){
    cout<<st.top();
    st.pop();
  }
  return 0;
}