Level order bottom

LevelOrderBottom.java

package com.interviewbytes.trees;

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

public class LevelOrderBottom {

    public List<List<Integer>> levelOrderBottom(TreeNode root) {

        LinkedList<List<Integer>> deque = new LinkedList<>();
        if (root == null) return deque;

        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);

        while (!queue.isEmpty()) {
            List<Integer> level = new ArrayList<>();
            int size = queue.size();
            while (size > 0) {
                TreeNode n = queue.poll();
                level.add(n.val);
                if (n.left != null) queue.offer(n.left);
                if (n.right != null) queue.offer(n.right);
                size--;
            }

            deque.offerFirst(level);
        }

        return deque;
    }
}

TreeNode.java

package com.interviewbytes.trees;

public class TreeNode {

    int val;
    TreeNode left;
    TreeNode right;

    TreeNode(int x) {
        val = x;
    }
}