Postfix to Prefix Conversion using Stack in C (C Program)

#include<stdio.h>
#include<string.h>
#include<stdlib.h>

#define MAX 20

char str[MAX], stack[MAX];
int top = -1;

void push (char c)
{
  stack[++top] = c;
}

char pop ()
{
  return stack[top--];
}

// A utility function to check if the given character is operand 
int checkIfOperand (char ch)
{
  return (ch >= 'a' && ch <= 'z') || (ch >= 'A' && ch <= 'Z');
}

//function to check if it is an operator
int isOperator (char x)
{
  switch (x)
    {
    case '+':
    case '-':
    case '/':
    case '*':
      return 1;
    }
  return 0;
}

void postfixToprfix ()
{
  int n, i, j = 0;
  char c[20];
  char a, b, op;

  printf ("Enter the postfix expression\n");
  scanf ("%s", str);

  n = strlen (str);

  for (i = 0; i < MAX; i++)
    stack[i] = '\0';
  printf ("Prefix expression is: ");

  for (i = n - 1; i >= 0; i--)
    {
      if (isOperator (str[i]))
	{
	  push (str[i]);
	}
      else
	{
	  c[j++] = str[i];
	  while ((top != -1) && (stack[top] == '#'))
	    {
	      a = pop ();
	      c[j++] = pop ();
	    }
	  push ('#');
	}
    }
  c[j] = '\0';

  i = 0;
  j = strlen (c) - 1;
  char d[20];

  while (c[i] != '\0')
    {
      d[j--] = c[i++];
    }

  printf ("%s\n", d);

}

int main ()
{
  postfixToprfix ();

  return 0;
}