//102.二叉树的层序遍历
class Solution {
  public List<List<Integer>> resList = new ArrayList<List<Integer>>();
    public List<List<Integer>> levelOrder(TreeNode root) {
        //checkFun01(root,0);
        checkFun02(root);
        return resList;
    }
    
    // BFS--迭代方式--辅助队列
    public void checkFun02(TreeNode node) {
        if (node == null)  return;
        Queue<TreeNode> que = new LinkedList<TreeNode>();
        que.offer(node);
        while(!que.isEmpty()) {
            List<Integer> itemList = new ArrayList<Integer>();
            int len = que.size();
            while(len--) {
                TreeNode tmpNode = que.poll();
                itemList.add(tmpNode.val);
                if (tmpNode.left != null)  que.offer(tmpNode.left);
                if (tmpNode.right != null) que.offer(tmpNode.right);
            }
            resList.add(itemList);
        }
    }
}