LL_add_at.java

import java.io.*;

// import java.util.*;

public class LL_add_at {
  public static class Node {
    int data;
    Node next;
  }

  public static class LinkedList {
    Node head;
    Node tail;
    int size;

    void addLast(int val) {
      Node temp = new Node();
      temp.data = val;
      temp.next = null;

      if (size == 0) {
        head = tail = temp;
      } else {
        tail.next = temp;
        tail = temp;
      }

      size++;
    }

    public int size() {
      return size;
    }

    public void display() {
      for (Node temp = head; temp != null; temp = temp.next) {
        System.out.print(temp.data + " ");
      }
      System.out.println();
    }

    public void removeFirst() {
      if (size == 0) {
        System.out.println("List is empty");
      } else if (size == 1) {
        head = tail = null;
        size = 0;
      } else {
        head = head.next;
        size--;
      }
    }

    public int getFirst() {
      if (size == 0) {
        System.out.println("List is empty");
        return -1;
      } else {
        return head.data;
      }
    }

    public int getLast() {
      if (size == 0) {
        System.out.println("List is empty");
        return -1;
      } else {
        return tail.data;
      }
    }

    public int getAt(int idx) {
      if (size == 0) {
        System.out.println("List is empty");
        return -1;
      } else if (idx < 0 || idx >= size) {
        System.out.println("Invalid arguments");
        return -1;
      } else {
        Node temp = head;
        for (int i = 0; i < idx; i++) {
          temp = temp.next;
        }
        return temp.data;
      }
    }

    public void addFirst(int val) {
      Node temp = new Node();
      temp.data = val;
      temp.next = head;
      head = temp;

      if (size == 0) {
        tail = temp;
      }

      size++;
    }

    public void addAt(int idx, int val) {
        if (idx < 0 || idx > this.size) {
            System.out.println("Invalid arguments");
        }
        else if (idx == 0) {     //point of mistake  
            addFirst(val);
        }
        else if (idx == size) {
            addLast(val);
        }
        else {
            Node new_node = new Node();
            new_node.data = val;

            Node temp = head;
            for (int i = 0; i < idx - 1; i++) {
                temp = temp.next;
            }

            new_node.next = temp.next;
            temp.next = new_node;
            size++;

        }
    }
  }

  public static void main(String[] args) throws Exception {
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    LinkedList list = new LinkedList();

    String str = br.readLine();
    while (str.equals("quit") == false) {
      if (str.startsWith("addLast")) {
        int val = Integer.parseInt(str.split(" ")[1]);
        list.addLast(val);
      } else if (str.startsWith("size")) {
        System.out.println(list.size());
      } else if (str.startsWith("display")) {
        list.display();
      } else if (str.startsWith("removeFirst")) {
        list.removeFirst();
      } else if (str.startsWith("getFirst")) {
        int val = list.getFirst();
        if (val != -1) {
          System.out.println(val);
        }
      } else if (str.startsWith("getLast")) {
        int val = list.getLast();
        if (val != -1) {
          System.out.println(val);
        }
      } else if (str.startsWith("getAt")) {
        int idx = Integer.parseInt(str.split(" ")[1]);
        int val = list.getAt(idx);
        if (val != -1) {
          System.out.println(val);
        }
      } else if (str.startsWith("addFirst")) {
        int val = Integer.parseInt(str.split(" ")[1]);
        list.addFirst(val);
      } else if (str.startsWith("addAt")) {
        int idx = Integer.parseInt(str.split(" ")[1]);
        int val = Integer.parseInt(str.split(" ")[2]);
        list.addAt(idx, val);
      }
      str = br.readLine();
    }
  }
}