102 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,15,7],?
    3
   /   9  20
    /     15   7

return its level order traversal as:?
[
  [3],[9,20],[15,7]
]




class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
      List<List<Integer>> result = new ArrayList<>();
      if(root == null) return result;
      Queue<TreeNode> queue = new LinkedList<>(); // queue,stack interface??
      queue.offer(root);
      while(!queue.isEmpty()){
        int size = queue.size();
        List<Integer> list = new ArrayList<>(); // create a new list here before the size 
        for(int i = 0; i < size; i++){
          TreeNode cur = queue.poll();
          list.add(cur.val);
          if(cur.left != null) queue.offer(cur.left);
          if(cur.right != null) queue.offer(cur.right);
        }
        result.add(list);
    }
    return result;
    }
}


// stack,push,pop
// queue. Offer,poll