Remove unbalanced parentheses from the string

Remove unbalanced parentheses from the string.

For example : If the input string is ((abc)((def))
                      Output string is (abc)((def)) , the first parentheses is unbalanced and removed.




Implementation:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX 100

/* structure of the stack */
struct stack
{
 int top;
 char items[MAX];
};

/* push function to push an element in the top of the stack */
void push(struct stack *s, char data)
{
 s->top++;
 s->items[s->top]=data;
}

/* pop function to pop an element from the top of the stack */
void pop(struct stack *s)
{
 s->top--;
}

int main(void) {
 char *str;
 int i,index=0;
 
 /* structure object */
 struct stack s;
 
 /* set the top of the stack as -1 initially */
 s.top=-1;
 
 /* allocate memory for the input string */
 str=(char*)malloc(sizeof(char)*256);
 
 /* get the input string from the user */
 scanf("%s",str);
 
 i=0;
 
 /* loop until we reach the end of the string */
 while(*(str+i))
 {
  
  /* Push all the characters onto the stack until we visit ')' character */
  if(*(str+i)!=')')
  {
   push(&s,*(str+i));
   index++;
  }
  else
  {
   /* If we see a ')' pop all the elements from the stack until we see '(' */
   while(s.items[s.top]!='(' && s.top!=-1)
   {
    pop(&s);
    index--;
   }
   
   /* pop the '(' character too */
   pop(&s);
   index--;
  }
  i++;
 }
 
 /* if stack is not empty, there must be an unbalanced parentheses left in the stack */
 if(s.top!=-1)
 {
  i=0;
  /* index pointing to the index of that unbalanced parentheses */
  while(*(str+i))
  {
   
   /* print all the characters of the string, except the one index pointing to */
   if(i==index)
   {
    i++;
    continue;
   }
   printf("%c",*(str+i));
   i++;
  }
 }
 
 return 0;
}