Infix to Postfix

#include <stdio.h>
#include <ctype.h>
#define MAX_SIZE 20

int stack[MAX_SIZE];
int top = -1;

int isEmpty() {
    return top == -1;
}

void push(int item) {
    if(top == MAX_SIZE-1) {
        printf("Error: Stack Overflow\n");
        return;
    }
    stack[++top] = item;
}

char pop() {
    if(isEmpty()) {
        printf("Error: Stack is Empty\n");
        return '$';
    }
    return stack[top--];
}

int precedence(char symbol) {
    if(symbol == '(') {
        return 0;
    } else if(symbol == '+' || symbol == '-') {
        return 1;
    } else if(symbol == '*' || symbol == '/') {
        return 2;
    } else if(symbol == '^') {
        return 3;
    }
}

int main() {

    char infix[20], x, temp;
    int i = 0;
    printf("Enter Infix Expression :\n");
    scanf("%s", infix);
    printf("Expression in Postfix Notation :\n");
    while(infix[i] != '\0') {
        x = infix[i];
        if(x == '(') {
            push(x);
        } else if(isalnum(x)) {
            printf("%c", x);
        } else if(x == ')') {
            while((temp = pop()) != '(') {
                printf("%c", temp);
            }
        } else {
            if(!isEmpty()) {
                while(precedence(stack[top]) >= precedence(x)) {
                    printf("%c", pop());
                }
            }
            push(x);
        }
        i++;
    }
    while(!isEmpty()) {
        printf("%c", pop());
    }
    printf("\n");

    return 0;
}