Getting NullPointerException whille inserting element in BST

Question

I am trying to insert element in Binary Search Tree with iterative method but i am getting NullPointerException and i am not able to figure out why this error is getting. I tried changing the loop and checking the temp but i didnt get what is happening wrong there. EDIT:- I have added the whole Code.

     import java.util.*;
  import java.io.*;

  class Node {
      Node left;
      Node right;
      int data;

      Node(int data) {
          this.data = data;
          left = null;
          right = null;
      }
  }

  class Solution {

      public static void preOrder(Node root) {

          if (root == null)
              return;

          System.out.print(root.data + " ");
          preOrder(root.left);
          preOrder(root.right);

      }

      /* Node is defined as :
      class Node 
         int data;
         Node left;
         Node right;

         */

      public static Node insert(Node root, int data) {

          Node inserter = new Node(data);
          Node temp = root;
          while (true) {
              if (inserter.data <= temp.data) {
                  if (temp.left == null) {
                      temp.left = inserter;
                      break;
                  } else
                      temp = temp.left;

              } else {
                  if (temp.right == null) {
                      temp.right = inserter;
                      break;
                  } else
                      temp = temp.right;

              }
          }

          return root;
      }

      public static void main(String[] args) {
          Scanner scan = new Scanner(System.in);
          int t = scan.nextInt();
          Node root = null;
          while (t-- > 0) {
              int data = scan.nextInt();
              root = insert(root, data);
          }
          scan.close();
          preOrder(root);
      }
  }

Answer 1

public static void main(String[] args) {
      Scanner scan = new Scanner(System.in);
      int t = scan.nextInt();
      Node root = null;
      while (t-- > 0) {
          int data = scan.nextInt();
          root = insert(root, data);
      }
      scan.close();
      preOrder(root);
  }

In main method you are passing root == null

You need to rewrite your insert method then it will work

public static Node insert(Node root, int data) {

          // check if root == null then initialize root and return it
          if(root == null){ 
               return new Node(data);
          }
          //--------

          Node inserter = new Node(data);
          Node temp = root;
          while (true) {
              if (inserter.data <= temp.data) {
                  if (temp.left == null) {
                      temp.left = inserter;
                      break;
                  } else
                      temp = temp.left;

              } else {
                  if (temp.right == null) {
                      temp.right = inserter;
                      break;
                  } else
                      temp = temp.right;

              }
          }

          return root;
      }

Answer 2

Your issue is in your main method:

  public static void main(String[] args) {
      Scanner scan = new Scanner(System.in);
      int t = scan.nextInt();
      Node root = null;
      while (t-- > 0) {
          int data = scan.nextInt();
          root = insert(root, data);
      }
      scan.close();
      preOrder(root);
  }

You initialize Node root = null, and the first time you enter your while loop, root will still be null when passing it to the insert method: root = insert(root, data);

public static Node insert(Node root, int data) {
  Node inserter = new Node(data);
  Node temp = root;
  while (true) {
    if (inserter.data <= temp.data) {
    ...

So the first time you enter the insert method, temp will be set to null, giving a nullpointer on the temp.data in the if (inserter.data <= temp.data) line.