Checking if brackets are matching in an expression

#include<stdio.h>
#include<ctype.h>
#include<stdlib.h>
struct node
{
        char data;
        struct node *next;
} *start=NULL;

void push(char);
char pop(void);
void print();

void main()
{
    char in='\0';
    while(in!='\n')
    {
        in=getchar();

        if(in =='(' || in=='[' || in=='{')
        {
            push(in);
//          print();
            continue;
        }
        else if (in ==')')
        {
            if((pop())+1==in)
            {
//              print();
                continue;
            }
            else
            {
                push(in-1);
                continue;
            }
        }

        else if (in ==']')
                {
                        if((pop())+2==in)
                        {
//              print();
                                continue;
                        }
            else
            {
                push(in-2);
                continue;
            }
                }

        else if (in =='}')
                {
                        if((pop())+2==in)
                        {
//              print();
                                continue;
                        }
            else
            {
                push(in-2);
                continue;
            }
                }

        else if(isalnum(in) || in=='+' || in=='-' || in=='*' || in=='/')
        {
//          print();
            continue;
        }
        else if(in!='\n')
        {
            printf("\nInvalid Expression\n");
            exit(1);
        }



    }

    if(start!=NULL)
    {
        printf("\nExpression is invalid\n");
        exit(1);
    }

    printf("\nExpression is valid\n");

}

void push(char element)
{

        struct node *new;
        new=(struct node *) (malloc(sizeof(struct node)));

    new->data=element;

        new->next=NULL;
        if(start==NULL)
        {
                start=new;

                return;
        }
        new->next=start;

        start=new;
}

char pop()
 {
    struct node *p;

    if(start==NULL) //if empty
     {
        return('!');
     }
     else
     {
        p=start;
        start=start->next;

        return(p->data);
     }
 }

void print()
{
        if(start==NULL)
        {
                printf("\nNo data in Linked List\n");
                return;
        }

        struct node *p;
        p=start;
        printf("\nLinked list: \n");

        while(p!=NULL)
        {
        printf("\n%c",p->data);
        p=p->next;
        }
}