Balanced Brackets Checker always gives incorrect output - c

Question

I have created a function which uses Linked List to check whether an expression is balanced or not. A balanced expression has no. of opening brackets equal to no. of closing brackets.
But the function Bracket Balancing always gives "unbalanced" as the output.
CODE:
#include <stdio.h>
#include <stdlib.h>
struct LL {
char data;
struct LL *next;
};
int isEmpty(struct LL *top) {
if (top == NULL) {
return 1;
}
else {
return 0;
}
}
int isFull(struct LL *top) {
struct LL *n = malloc(sizeof(struct LL *));
if (n == NULL) {
return 1;
}
else {
return 0;
}
}
struct LL *push(struct LL *top, char x) {
if (isFull(top)) {
printf("Stack Overflow\n");
}
else {
struct LL *n = malloc(sizeof(struct LL));
n->data = x;
n->next = top;
top = n;
}
return top;
}
struct LL *pop(struct LL *top) {
if (isEmpty(top)) {
printf("Stack Underflow\n");
}
else {
struct LL *n = malloc(sizeof(struct LL));
n = top;
top = top->next;
free(n);
}
return top;
}
int BracketBalancing (char *exp) {
struct LL *top = malloc(sizeof(struct LL));
top->next = NULL;
for (int i = 0; exp[i] != '\0'; i++) {
if (exp[i] == '(') {
push(top, exp[i]);
}
else if (exp[i] == ')') {
if (isEmpty(top)) {
return 0;
}
pop(top);
}
}
if (isEmpty(top)) {
return 1;
}
else {
return 0;
}
}
MAIN:
int main(int argc, char const *argv[]) {
int n;
char *expression = (char *)malloc(sizeof(char));
printf("Enter the length of the expression for Bracket Balancing\n");
scanf("%d", &n);
printf("Enter the expression for Bracket Balancing\n");
for (int i = 0; i < n; i++) {
scanf("%c ", &expression[i]);
}
getchar();
if (BracketBalancing(expression)) {
printf("The expression is balanced\n");
}
else if (!BracketBalancing(expression)) {
printf("This expression is unbalanced\n");
}
return 0;
}
Example:
Input:
Enter the length of the expression for Bracket Balancing
4
Enter the expression for Bracket Balancing
1+()
Output:
This expression is unbalanced
In the above example, Despite the expression being balanced the output generated is "This expression is unbalanced".
Please correct my code.

Answer 1

This is how you initialize your list:
struct LL *top = malloc(sizeof(struct LL));
top->next = NULL;
And this is isEmpty():
int isEmpty(struct LL *top)
{
if (top == NULL)
{
return 1;
}
else
{
return 0;
}
}
But: top starts with a value != NULL, so isEmtpy() will not return 1, although our list should be empty in the beginning.
Your implementation of push() should work fine when you pass NULL, so you can just initialize struct LL *top = NULL; instead of creating the first element rightaway.
there other bugs in your code, e.g.:
in pop() you do
struct LL *n = malloc(sizeof(struct LL));
n = top;
thus, the result of malloc() is directly overwritten() in the next line
in isFull() you produce a memory leak as you call malloc() and never use or free() the buffer returned. That function doesn't make sense anyway, just check the result of malloc()s where your really want to use the buffer returned.
** Edit **
What I haven't seen before, you also never use the return value of push() and pop() so the new top determined by these function is lost. Replace push(top, ...); by top = push(top,...); and pop(top); by top = pop(top);