二叉树的层次遍历

public static List<List<Integer>> levelOrder(TreeNode root) {
        if (root==null)return null;
        List<List<Integer>> lists = new ArrayList<List<Integer>>();
        List<Integer> List = new ArrayList<Integer>();
        List.add(root.val);
        lists.add(List);
        List<TreeNode> treeNodes = new ArrayList<TreeNode>();
        treeNodes.add(root);
         saveData(treeNodes,lists);
        return lists;
    }

    public static void saveData(List<TreeNode> treeNodes, List<List<Integer>> lists){
        if(treeNodes.isEmpty())return ;
        List<TreeNode> nodeList = new ArrayList<TreeNode>();
        List<Integer> list = new ArrayList<Integer>();
        for(TreeNode treeNode:treeNodes){
            if(treeNode.left!=null){
                list.add(treeNode.left.val);
                nodeList.add(treeNode.left);
            }
            if(treeNode.right!=null){
                list.add(treeNode.right.val);
                nodeList.add(treeNode.right);
            }
        }
        if(!list.isEmpty())lists.add(list);
        saveData(nodeList,lists);
    }