从上到下按层打印二叉树

从上到下按层打印二叉树，同一层的节点按从左到右的顺序打印，每一层打印到一行。

例如:
给定二叉树: [3,9,20,null,null,15,7],

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    List<List<Integer>> res = new ArrayList();
    public List<List<Integer>> levelOrder(TreeNode root){
        recur(root,0);
        return res;
    }
    public void recur(TreeNode root,int k){
        if(root != null){
            if(res.size() <= k)
                res.add(new ArrayList());
            res.get(k).add(root.val);
            recur(root.left,k+1);
            recur(root.right,k+1);
        }
    }
}