Binary Tree Level Order Traversal II

https://leetcode.com/problems/binary-tree-level-order-traversal-ii

Given a binary tree, return the bottom-up level order traversal of its nodes' values. (ie, from left to right, level by level from leaf to root).

For example:
Given binary tree [3,9,20,null,null,15,7],
    3
   / \
  9  20
    /  \
   15   7
return its bottom-up level order traversal as:
[
  [15,7],
  [9,20],
  [3]
]

Implementation 1 :

/**
 * Definition for a binary tree node.
 * public 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;
 *     }
 * }
 */
class Solution {
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        if(root == null)
           return res;
        Queue<TreeNode> q = new LinkedList<>();
        q.offer(root);
        while(!q.isEmpty()) {
            int size = q.size();
            List<Integer> level = new ArrayList<>();
            for(int i = 1; i <= size; i++) {
                TreeNode current = q.poll();
                level.add(current.val);
                if(current.left != null)
                    q.offer(current.left);
                if(current.right != null)
                    q.offer(current.right);
            }
            res.add(level);
        }
        Collections.reverse(res);
        return res;
    }
}

Implementation 2 :

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        List<List<Integer>> result = new ArrayList<List<Integer>>();
        if(root == null){
            return result;
        }
        Queue<TreeNode> queue = new ArrayDeque<TreeNode>();
        queue.offer(root);
        while(!queue.isEmpty()){
            int size = queue.size();
            List<Integer> level = new ArrayList<Integer>();
            for(int i = 0; i < size; i++){
                TreeNode node = queue.poll();
                level.add(node.val);
                if(node.left != null)
                    queue.offer(node.left);
                if(node.right != null)
                    queue.offer(node.right);
            }
          result.add(0,level);
        }
        return result;
    }
}

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