InfixToPostfix in c

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

#define MAX 100

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

void push(char c) {
    if (top == MAX - 1) {
        printf("Stack Overflow\n");
        return;
    }
    stack[++top] = c;
}

char pop() {
    if (top == -1) {
        printf("Stack Underflow\n");
        return -1;
    }
    return stack[top--];
}

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

int isOperator(char c) {
    return (c == '+' || c == '-' || c == '*' || c == '/' || c == '^');
}

void infixToPostfix(char infix[]) {
    char postfix[MAX];
    int j = 0;
    for (int i = 0; i < strlen(infix); i++) {
        char c = infix[i];

        if (isalnum(c)) {
            postfix[j++] = c;
        } 
        else if (c == '(') {
            push(c);
        } 
        else if (c == ')') {
            while (top != -1 && stack[top] != '(') {
                postfix[j++] = pop();
            }
            pop(); // remove '('
        } 
        else if (isOperator(c)) {
            while (top != -1 && precedence(stack[top]) >= precedence(c)) {
                postfix[j++] = pop();
            }
            push(c);
        }
    }

    while (top != -1) {
        postfix[j++] = pop();
    }
    postfix[j] = '\0';

    printf("Postfix Expression: %s\n", postfix);
}

int main() {
    char infix[MAX];
    printf("Enter an infix expression: ");
    scanf("%s", infix);

    infixToPostfix(infix);

    return 0;
}