深搜：
public static ArrayList<ArrayList<TreeNode>> getRes(TreeNode root){
ArrayList<ArrayList<TreeNode>> res = new ArrayList<ArrayList<TreeNode>>();
getRes(res,root,0);
return res;
}
public static void getRes(ArrayList<ArrayList<TreeNode>> res,TreeNode root, int level){
if( root== null){
return;
}
ArrayList<TreeNode> newlist ;
if( res.get( level).size()== level){
newlist = new ArrayList<TreeNode>();
} else{
newlist = res.get( level);
}
newlist.add( root);
getRes(res,root.leftNode,level+1);
getRes(res,root.rightNode,level+1);
}
