BTlevelOrderTraversal.java

/*
Given the root of a binary tree, return the level order traversal of its nodes' values. (i.e., from left to right, level by level).

Input: root = [3,9,20,null,null,15,7]
Output: [[3],[9,20],[15,7]]
 */
import java.util.*;

//import
public class BTlevelOrderTraversal {
    public static void main(String[] args) {

        TreeNode root= new TreeNode(3);
        root.left=new TreeNode(9);
        root.right=new TreeNode(20);
        root.left.left=null;
        root.left.right=null;
        root.right.left=new TreeNode(15);
        root.right.right=new TreeNode(7);

        List<List<Integer>> ans=levelOrderBottom(root);
        System.out.println(ans);



    }
    public static List<List<Integer>> levelOrderBottom(TreeNode root) {
        List<List<Integer>> result=new ArrayList<>();
        if(root == null){
            return result;
        }
        Queue<TreeNode> queue=new LinkedList<>();
        queue.offer(root);

        while(!queue.isEmpty()){
            int levelsize=queue.size();
            List<Integer> currentlevel=new ArrayList<>(levelsize);
            for(int i=0; i<levelsize; i++){
                TreeNode currentNode=queue.poll();
                currentlevel.add(currentNode.val);
                if(currentNode.left != null){
                    queue.offer(currentNode.left);
                }
                if(currentNode.right != null){
                    queue.offer(currentNode.right);
                }
            }
            result.add(currentlevel);

        }
//        Collections.reverse(result);
        return result;


    }
}
// Data structure to store the nodes of the tree
class TreeNode {
      int val;
      TreeNode left;
      TreeNode right;
      TreeNode() {}
      TreeNode(int val) {
          this.val=val;
//          val=data;
//          left=right=null;

      }
      TreeNode(int val, TreeNode left, TreeNode right) {
          this.val = val;
          this.left = left;
          this.right = right;
      }
  }