Convert an Infix expression to Prefix form using Linked List

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

#define MAX 50

struct infix
{
    char target[MAX] ;
    char stack[MAX] ;
    char *s, *t ;
    int top, l ;
} ;

void initinfix ( struct infix * ) ;
void setexpr ( struct infix *, char * ) ;
void push ( struct infix *, char ) ;
char pop ( struct infix * ) ;
void convert ( struct infix * ) ;
int priority ( char c ) ;
void show ( struct infix ) ;

void main( )
{
    struct infix q ;
    char expr[MAX] ;

    clrscr( ) ;

    initinfix ( &q ) ;

    printf ( "\nEnter an expression in infix form: " ) ;
    gets ( expr ) ;

    setexpr ( &q, expr ) ;
    convert ( &q ) ;

    printf ( "The Prefix expression is: " ) ;
    show ( q ) ;

    getch( ) ;
}

/* initializes elements of structure variable */
void initinfix ( struct infix *pq )
{
    pq -> top = -1 ;
    strcpy ( pq -> target, "" ) ;
    strcpy ( pq -> stack, "" ) ;
    pq -> l = 0 ;
}

/* reverses the given expression */
void setexpr ( struct infix *pq, char *str )
{
    pq -> s = str ;
    strrev ( pq -> s ) ;
    pq -> l = strlen ( pq -> s ) ;
    *( pq -> target + pq -> l ) = '\0' ;
    pq -> t = pq -> target + ( pq -> l - 1 ) ;
}

/* adds operator to the stack */
void push ( struct infix *pq, char c )
{
    if ( pq -> top == MAX - 1 )
        printf ( "\nStack is full.\n" ) ;
    else
    {
        pq -> top++ ;
        pq -> stack[pq -> top] = c ;
    }
}

/* pops an operator from the stack */
char pop ( struct infix *pq )
{
    if ( pq -> top == -1 )
    {
        printf ( "Stack is empty\n" ) ;
            return -1 ;
    }
    else
    {
        char item = pq -> stack[pq -> top] ;
        pq -> top-- ;
        return item ;
    }
}

/* converts the infix expr. to prefix form */
void convert ( struct infix *pq )
{
    char opr ;

    while ( *( pq -> s )  )
    {
        if ( *( pq -> s )  == ' ' || *( pq -> s )  == '\t' )
        {
            pq -> s++    ;
            continue ;
        }

        if ( isdigit ( *( pq -> s )  ) || isalpha ( *( pq -> s )  ) )
        {
            while ( isdigit ( *( pq -> s )  ) || isalpha ( *( pq -> s )  ) )
            {
                *( pq -> t ) = *( pq -> s )  ;
                pq -> s++   ;
                pq -> t-- ;
            }
        }

        if ( *( pq -> s )  == ')' )
        {
            push ( pq, *( pq -> s ) ) ;
            pq -> s++    ;
        }

        if ( *( pq -> s )  == '*' || *( pq -> s )  == '+' || *( pq -> s )  == '/' ||
             *( pq -> s )  == '%' || *( pq -> s )  == '-' || *( pq -> s )  == '$' )
        {
            if ( pq -> top != -1 )
            {
                opr = pop ( pq ) ;

                while ( priority ( opr ) > priority ( *( pq -> s )  ) )
                {
                    *( pq -> t ) = opr ;
                    pq -> t-- ;
                    opr = pop ( pq ) ;
                }
                push ( pq, opr ) ;
                push ( pq, *( pq -> s )  ) ;
            }
            else
                push ( pq, *( pq -> s ) ) ;
            pq -> s++    ;
        }

        if ( *( pq -> s )  == '(' )
        {
            opr = pop ( pq ) ;
            while ( opr != ')' )
            {
                *( pq -> t ) = opr ;
                pq -> t-- ;
                opr =  pop ( pq ) ;
            }
            pq -> s++    ;
        }
    }

    while ( pq -> top != -1 )
    {
        opr = pop ( pq ) ;
        *( pq -> t ) = opr ;
        pq -> t-- ;
    }
    pq -> t++ ;
}

/* returns the priotity of the operator */
int priority ( char c )
{
    if ( c == '$' )
        return 3 ;
    if ( c == '*' || c == '/' || c == '%' )
        return 2 ;
    else
    {
        if ( c == '+' || c == '-' )
            return 1 ;
        elsereturn 0 ;
    }
}

/* displays the prefix form of given expr. */
void show ( struct infix pq )
{
    while ( *( pq.t ) )
    {
        printf ( " %c", *( pq.t ) ) ;
        pq.t++ ;
    }
}