struct Stack
{
    char data;
    struct Stack *next;
}*top = 0;

void push(char x)
{
    struct Stack *t;
    t = (struct Stack *)malloc(sizeof(struct Stack));
    if(t==0)
        return;
    else
    {
        t->data = x;
        t->next = top;
        top = t;
    }
}

char pop()
{
    struct Stack *t;
    char x=-1;
    if(top==0)
        return x;
    else
    {
        t = top;
        top = top->next;
        x = t->data;
        free(t);
        return x;
    }
}


char isTop()
{
    char x = -1;
    if(top==0)
        return x;
    return top->data;
}


bool isValid(char * exp){
for(int i=0;exp[i]!='\0';i++)
{
    if(exp[i]=='{'||exp[i]=='('||exp[i]=='[')
        push(exp[i]);
    else if((exp[i]==')' && isTop()!='(') ||
           (exp[i]=='}'&& isTop()!='{') ||
           (exp[i]==']'&& isTop()!='['))
        return false;
    else if(exp[i]==')' || exp[i]==']' || exp[i]=='}')
    {
        if(top==0) // stack is empty..
            return false;
        pop();
    }
}
    
if(top==0)
    return true;
return false;    
}