Infix to Postfix

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
char stack[100];
int top = -1;

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

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

int priority(char element)
{
    if(element==-1)
        return 0;
    if(element=='(')
        return 1;
    if(element=='+' || element=='-')
        return 2;
    if(element== '*' || element == '/')
        return 3;
}

void main()
{
    char infix[100], postfix[100], ch, element;
    int i = 0, j = 0;
    printf("Expression:\n");
    scanf("%s", infix);
    push(-1);
    while ((ch = infix[i++]) != '\0')
    {
        if (ch == '(')
            push(ch);
        else if (isalnum(ch))
            postfix[j++] = ch;
        else if (ch == ')')
        {
            while (stack[top] != '(')
                postfix[j++] = pop();
        }
        else
        {
            while (priority(stack[top]) >= priority(ch))
                postfix[j++] = pop();
        }
    }
    while (stack[top] != -1)
        postfix[j++] = pop();
    postfix[j] = '\0';
    printf("\nInfix: %s  \nPostfix: %s\n", infix, postfix);
    getchar();
    getchar();
    return 0;
}