把二叉树打印成多行 python

class Solution:
    # 返回二维列表[[1,2],[4,5]]
    def Print(self, pRoot):
        # write code here
        if not pRoot:
            return []
        tree = [pRoot]
        result = []
        while tree:
            subtree = []
            row = []
            for i in tree:
                row.append(i.val)
                if i.left:
                    subtree.append(i.left)
                if i.right:
                    subtree.append(i.right)
            result.append(row)
            tree = subtree
        return result