Binary Tree Level Order Traversal

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

For example:
Given binary tree [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its level order traversal as:

[
  [3],
  [9,20],
  [15,7]
]

Ans:

public List<List<Integer>> levelOrder(TreeNode root) {
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        List<List<Integer>> result = new LinkedList<List<Integer>>();
        
        if(root == null) {
            return result;
        }
        
        queue.add(root);
        while(!queue.isEmpty()){
            int size = queue.size();
            List<Integer> tempList = new LinkedList<Integer>();
            for(int i=0; i<size; i++) {
                TreeNode temp = queue.poll();
                if(temp.left != null)  {
                    queue.add(temp.left);
                }
                if(temp.right != null) {
                    queue.add(temp.right);
                }
                tempList.add(temp.val);
            }
            result.add(tempList);
        }
        return result;
    }

OR

public List<List<Integer>> levelOrder(TreeNode root) {
        
        List<List<Integer>> levelOrder = new ArrayList<List<Integer>>();
        List<Integer> levelList = new ArrayList<Integer>();
        int depth = depth(root);
        for(int i=1; i<=depth; i++) {
            levelOrder.add(i-1, addLevel(root, i, new ArrayList<Integer>()));
        }
        return levelOrder;
    }
    
    private int depth(TreeNode T) {
        if(T==null) {
            return 0;
        }
        return Math.max(depth(T.left), depth(T.right))+1;
    }
    
    private ArrayList addLevel(TreeNode T, int level, ArrayList<Integer> levelList) {
        if(T==null){
            return null;
        }
        if(level == 1) {
            levelList.add(T.val);
        }
        addLevel(T.left, level-1, levelList);
        addLevel(T.right, level-1, levelList);
        
        return levelList;
    }