#include<stdio.h>
#include<conio.h>
#include<ctype.h>

#define MAX 50

enum operators
    {
        multiply=0, divide=0, add=1, subtract=1, leftparen=2
    }op;

    int push(char[], char, int);
    int pop(char[], int*);
    int checkStack(char[],int*,char[],int);

    void main()
    {
        char infix[MAX]="", postfix[MAX]="", Stack[MAX]="", element;
        int p=-1,i=0,disp=0,top=-1,oper=0;



        //Get the infix expression as input
        puts("Enter the infix expression with single character variables and operators");
        gets(infix);

        top=push(Stack,'(',top);
        //Insert a dummy left parenthesis to write all the remaining operators in the stack

        for(i=0;infix[i] != '\0';i++)//For each character in the infix expression
    {
        if(isalpha(infix[i]))//Write operands in the postfix expression
    {
        postfix[++p] = infix[i];
    }
    else//If operator
    {
      switch(infix[i])
      {
            case '*':
                          op = multiply;
                          oper=1;
                          break;
            case '/':
                          op = divide;
                          oper=1;
                          break;
            case '+':
                          op = add;
                          oper=1;
                          break;
            case '-':
                          op = subtract;
                          oper=1;
                          break;
            case '(':
                          top=push(Stack,'(',top);//Push left parenthesis into the stack
                          break;
            case ')':
                          while((element=pop(Stack,&top;)) != '(')
                    //Pop from stack and write to postfix expression till u encounter a left parenthesis
                          {
                            postfix[++p] = element;
                          }//while
                          //Delete the left parenthesis
                          break;
      }//switch

      if(oper==1)//Perform only for operators
      {
            //Check with the operator at the top of the stack
            p=checkStack(Stack,&top;,postfix,p);
            top=push(Stack,infix[i],top);//Push operator to the stack
            oper=0;
      }
    }//else
  }//for

  while((element=pop(Stack,&top;)) != '(' )
  //Use the first pushed '(' to pop the remaining elements in the stack
  {
    postfix[++p] = element;
  }//while

  for(disp=0;disp<=p;disp++)//Print the postfix expression
    putchar(postfix[disp]);

  getch();
}

int checkStack(char Stack[],int *top, char postfix[], int p)
{
    enum operators opStack;

    do
    {
      switch(Stack[(*top)])
      {
            case '*':
                          opStack = multiply;
                          break;
            case '/':
                          opStack = divide;
                          break;
            case '+':
                          opStack = add;
                          break;
            case '-':
                          opStack = subtract;
                          break;
            case '(':
                          opStack = leftparen;
      }//switch

      if(op >= opStack)
      //If the operator at the top of the stack is of equal or higher precedence
            postfix[++p] = pop(Stack,top);//Pop and write to the postfix expression
      }while(op >= opStack);

      return(p);
}


int push(char Stack[],char element,int top)
{
    if(top == MAX-1)
    printf("Stack Overflow");
    else
    {
        Stack[++top] = element;
    }
    return(top);
}

int pop(char Stack[],int *top)
{
    int element;
    if((*top) == -1)
    {
        printf("Stack Underflow");
        element = -999;
    }
    else
    element = Stack[(*top)--];
    return(element);
}