Balanced Paranthesis

#include <stdio.h>
#include <stdlib.h>

#define NULL 0

struct ListNode {
    int data;
    struct ListNode *next;
};

typedef struct ListNode LN;

class SinglyLinkedList {
  public:
    // constructor
    SinglyLinkedList() {
        head = NULL;
    }

    LN *head;

    void print() {
        LN *temp = head;
        while (temp != NULL) {
            printf("%d ", temp->data);
            temp = temp->next;
        }
        printf("\n");
    }

    void insert_first(int item) {
        LN *new_node = (LN *) malloc(sizeof(LN));
        new_node->data = item;
        new_node->next = head;
        head = new_node;
    }

    void remove_first() {
        if (head == NULL)
            return;
        LN *temp = head;
        head = head->next;
        free(temp);
    }
};

class Stack {
    SinglyLinkedList L;

  public:
    // constructor
    Stack() {
    }

    void push(int item) {
        L.insert_first(item);
    }

    void pop() {
        L.remove_first();
    }

    int peek() {
        if (L.head == NULL) {
            printf("Stack Underflow\n");
            return -1;
        }
        return L.head->data;
    }

    void print() {
        L.print();
    }
};

int main() {

    char str[100];
    gets(str);

    Stack s;

    int i = 0;
    int flag = 1;
    while (str[i] != '\0') {
        // write your code here
        if (str[i] == '(' || str[i] == '{' || str[i] == '[') {
            s.push(str[i]);
        } else {
            if (s.peek() == -1) {
                flag = 0;
                break;
            } else {
                if (s.peek() == '(' && str[i] != ')') {
                    flag = 0;
                    break;
                } else if (s.peek() == '{' && str[i] != '}') {
                    flag = 0;
                    break;
                } else if (s.peek() == '[' && str[i] != ']') {
                    flag = 0;
                    break;
                } else {
                    s.pop();
                }
            }
        }
        i++;
    }
    if (flag && s.peek() == -1) {
        printf("Balanced\n");
    } else {
        printf("Not Balanced\n");
    }

    return 0;
}