Infix to Postfix Using Stack

//Considered only for () type brackets
//Works for only valid Infix expression

#include<bits/stdc++.h>
using namespace std;
int precedence(char c)
{
    if(c=='^')
    return 3;
    else if(c=='*' || c=='/')
    return 2;
    else if(c=='+' || c=='-')
    return 1;
    return -1;
}
int main()
{
    
    string infix;
    cout<<"Enter Infix : "<<endl;
    cin >> infix;
    string postfix;
    stack<char>counter;
    counter.push('#');
    //Length of expression
    int len=infix.length();
    for(int i=0;i<len;i++)
    {
        //If operand find add directly to postfix
        if((infix[i]>=65 && infix[i]<=90) || (infix[i]>=97 && infix[i]<=122))
        {
           postfix+=infix[i];
            
        }
       else if(infix[i]=='(')
        {
           counter.push(infix[i]); //If opening bracket found add directly to stack
        }
        else if(infix[i]==')')
        {
            //If closing bracket is found pop stack untill opening bracket found and concatenate popped 
            // characters in postfix
            while(counter.top()!='#' && counter.top()!='(')
            {
                char c=counter.top();
                counter.pop();
                postfix+=c;
            }
            //delete latest opening bracket
            if(counter.top()=='(')
            {
                char c=counter.top();
                counter.pop();
            }
        }
        else{
            //If operator found pop top operators of precedence greater than equal to current and concatenate
            // to postfix
            while(counter.top()!='#' && precedence(infix[i])<=precedence(counter.top()))
            {
                char c=counter.top();
                counter.pop();
                postfix+=c;
            }
            counter.push(infix[i]);
        }
    }
    
    
    // concatenate all remaining operands
    while(counter.top() != '#') 
    { 
        char c = counter.top(); 
        counter.pop(); 
        postfix += c; 
    } 
    cout<<endl<<"Postfix"<<endl;  
    cout<<postfix;
    return 0;
}





// Explanation
/*

    *ALGORITHM*
   1-> If the scanned character is an operand, output it.
   2-> Else,
   …..1) If the precedence of the scanned operator is greater than the precedence of the operator in the stack(or the stack is empty or the stack contains a ‘(‘ ), push it.
   …..2) Else, Pop all the operators from the stack which are greater than or equal to in precedence than that of the scanned operator. After doing that Push the scanned operator to the stack. (If you encounter parenthesis while popping then stop there and push the scanned operator in the stack.)
   3-> If the scanned character is an ‘(‘, push it to the stack.
   4-> If the scanned character is an ‘)’, pop the stack and and output it until a ‘(‘ is encountered, and discard both the parenthesis.
   5-> Repeat steps 2-6 until infix expression is scanned.
   6-> Pop and output from the stack until it is not empty.
   7. Print the output

    consider the Infix : (A+B)+C*D/E
    len=11
    when i=0
        postfix="";
        counter= (
        
        i=1
        postfix="A"
        counter= (
        
        i=2
        postfix="A"
        counter= (+
        
        i=3
        postfix="AB"
        counter= (+
        
        i=4
        postfix="AB+"
        counter= 
        
        i=5
        postfix="AB+"
        counter= +
        
        i=6
        postfix="AB+C"
        counter= +
        
        i=7
        postfix="AB+C"
        counter= +*
        
        i=8
        postfix="AB+CD"
        counter= +*
        
        i=9
        postfix="AB+CD*"
        counter= +/
        
        i=10
        postfix="AB+CD*E"
        counter= +/
        
        i=11
        out
        
        postfix="AB+CD*E/+"
        counter = 
        
        
         FINAL : postfix="AB+CD*E/+"

*/