บทเรียน: Tree & Binary Search Tree

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

# inorder: ซ้าย -> ตัวเอง -> ขวา
def inorder(node):
    if not node:
        return
    inorder(node.left)
    print(node.val)
    inorder(node.right)

from collections import deque
def level_order(root):
    if not root:
        return []
    res, q = [], deque([root])
    while q:
        node = q.popleft()
        res.append(node.val)
        if node.left:  q.append(node.left)
        if node.right: q.append(node.right)
    return res