Binary Tree Level Order Traversal

Analysis:
Typical BFS, super easy.
Solution:

Queue<TreeNode> queue = new LinkedList<>();  
        List<List<Integer>> res = new ArrayList<>();  
        if (root == null) return res;  
        queue.offer(root);  

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