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

class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
       if(root==null) return new ArrayList<List<Integer>>();
        List<List<Integer>> ans  = new ArrayList<>();
        Deque<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        while (!queue.isEmpty()){
            int currentSize = queue.size();
            List<Integer> list = new ArrayList<>();
            for(int i=0;i<currentSize;i++){
                TreeNode temp = queue.pop();
                list.add(temp.val);
                if(temp.left!=null){
                    queue.offer(temp.left);
                }
                if(temp.right!=null){
                    queue.offer(temp.right);
                }
            }
            ans.add(list);
        }
        return ans;
    }
}