以下代码可以正确地按层换行输出二叉树的节点值

from collections import deque

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

def printByLevel(root):
    if not root:
        return
    q = deque([root])
    while q:
        for i in range(len(q)):
            cur = q.popleft()
            print(cur.val, end='')
            if cur.left:
                q.append(cur.left)
            if cur.right:
                q.append(cur.right)
        print()