32 - I. 从上到下打印二叉树

题目链接：https://leetcode-cn.com/problems/cong-shang-dao-xia-da-yin-er-cha-shu-lcof/
难度：中等

描述：
从上到下打印出二叉树的每个节点，同一层的节点按照从左到右的顺序打印。

题解

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
class Solution:
    def levelOrder(self, root: TreeNode) -> List[int]:
        ret = []
        q = deque()
        if root is not None:
            q.append(root)
        while q:
            size = len(q)
            while size > 0:
                cur = q.popleft()
                ret.append(cur.val)
                if cur.left is not None:
                    q.append(cur.left)
                if cur.right is not None:
                    q.append(cur.right)
                size -= 1
        return ret