Binary_Tree_Level_Order_Traversal_II.java

package com.leet_code;

import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.Queue;

public class Binary_Tree_Level_Order_Traversal_II {
    public static void main(String[] args) {

        TreeNode root = new TreeNode(3);

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

        System.out.println(levelOrderBottom(root));
    }
    public static class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;
        TreeNode() {}
        TreeNode(int val) { this.val = val; }
        TreeNode(int val, TreeNode left, TreeNode right) {
            this.val = val;
            this.left = left;
            this.right = right;
        }
    }
    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.add(root);
        while (!queue.isEmpty()) {
            int size=queue.size();
            List<Integer> arr=new ArrayList<>(size);
            for (int i=0;i<size;i++) {
               TreeNode node=queue.poll();
                arr.add(node.val);
                if (node.left != null) {
                    queue.add(node.left);
                }
                if (node.right != null) {
                    queue.add(node.right);
                }
            }
            result.addFirst(arr);
        }
        return result;
    }
}