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public class Test {
    public static void main(String[] args) {
        SinglyLinkedList list = new SinglyLinkedList(); // {null}
        list.insertToHead(5); // {5} -> {null}
        list.insertToTail(1); // {5} -> {1} -> {null}
        list.insertToHead(2); // {2} -> {5} -> {1} -> {null}
        list.insertToTail(3); // {2} -> {5} -> {1} -> {3} -> {null}
        list.deleteAt(3);
        list.deleteAt(5);
        list.deleteAt(1);
        list.deleteAt(1);
        list.deleteAt(1);
        list.deleteAt(2);
        list.deleteAt(2);
        list.traverse();
    }
}

class SinglyLinkedList {
    Node head;

    void insertToHead(int value) {
        Node newNode = new Node(value);
        newNode.next = head;
        head = newNode;
    }

    void insertToTail(int value) {
        Node newNode = new Node(value);

        if (head == null){
            head = newNode;
        } else {
            Node temp = head;
            while (temp.next != null) {
                temp = temp.next;
            }
            temp.next = newNode;
        }        
    }

    void insertAt(int data, int value) {
        Node temp = head;
        Node newNode = new Node(value);
        if (head == null) {
            head = newNode;
        } else {
            while(temp.next != null) {
                if (temp.data == data) {
                    newNode.next = temp.next;
                    temp.next = newNode;
                } 
                temp = temp.next;
            }
        }
    }

    void deleteHead() {
        if (head != null) {
            head = head.next;
        }
    }

    void deleteTail() {
        if (head == null) {
            // wala
        } else if (head.next == null) {
            head = head.next;
        } else {
            Node temp = head;
            Node prev = null;
            while (temp.next != null) {
                prev = temp;
                temp = temp.next;
            }
            prev.next = null;
        }
    }

    void deleteAt(int data) {
        Node temp = head;
        Node prev = null;

        if (head == null){
            System.out.println("list is empty");
        } else if (temp.data == data) {
            deleteHead();
        } else {
            while(temp.next != null) {
                if (temp.data == data) {
                    prev.next = temp.next;
                }
                prev = temp;
                temp = temp.next;
            }
            if (temp.data == data) {
                prev.next = temp.next;
            }
        }
    }

    void traverse() {
        Node temp = head;
        while (temp != null) {
            System.out.println(temp.data);
            temp = temp.next;
        }
    }
}

class Node {
    int data;
    Node next;

    Node(int data) {
        this.data = data;
    }
}