Unable to pass all test cases-> string problem with use of stack data structure in C

Question

Regarding this leetcode problem

My solution: I am pushing all characters in a string to a stack data structure. While pushing I keep track of the number of '(' and ')' using count, which increments for '(' and decrements for ')'. I try to maintain the count as zero by popping ')' from stack when it is negative. After going through the entire loop, if count is positive, then I try to remove '(' going over the stack. And I store the result in variable res.

This is above approach is not working for some test cases. Can you point out the issues with my code?

I have not tried to improve the performance yet. I just wanted to get this initial code working with all cases.

typedef struct stack
{
    char *data;
    int topIndex;
    int total;
}stack;

stack *st;

void stackPush(char x)
{
    st->topIndex++;
    st->data[st->topIndex] = x;
}

char stackPop()
{ 
    char a = 0;

    if(st->topIndex > -1)
    {
        a = st->data[st->topIndex];
        st->topIndex--;
    }
    return a;

}

char * minRemoveToMakeValid(char * s){
    
    int l = strlen(s);
    char r;
    char *res = (char *) malloc(sizeof(char)*(l+1));
    st = (stack*)malloc(sizeof(stack));
    st->data = (char*) malloc(sizeof(char)*(l));
    st->topIndex = -1;
    int count = 0;
    int j = 0;
    int u =0;

    // maintain count as zero, if negative try to pop.
    while(j < l)
    {
        if(s[j] == '(')
        {
            count++;
        }
        else if(s[j] == ')')
        {
            count--;    
        }
        stackPush(s[j]);
        if(count < 0)
        {
            count++;
            stackPop();
        }       
        j++;
    }

    u = st->topIndex;
    res[u+1] =  '\0';

    // if count is greater than zero, try to bring it down to zero by popping '('
    while ((st->topIndex > -1))
    {
        r = stackPop();

        if(((count > 0) && (r == '(')))
        {
            count--;
        }
        else
        {
            res[u] = r;
          
            u--;
        }
    }

    // wanted to initialize res to empty string in case the test case is "))(("
    if(u > st->topIndex)
    {
        res = "";
    }
    return res;
}