1. 剑指 Offer 32 - I. 从上到下打印二叉树


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# 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]:
        queue = [root]
        res = []
        while queue:
            cur = queue.pop()
            if cur:
                res.append(cur.val )
                queue = [cur.right,cur.left]+queue
        return res

2. 剑指 Offer 32 - II. 从上到下打印二叉树 II


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class Solution:
    def levelOrder(self, root: TreeNode) -> List[List[int]]:
        queue = [root]
        res = []
        while(queue):
            cur_res=[]
            for i in range(len(queue)): # 这句保证每次清空上一轮队列
                cur = queue.pop()
                if cur:
                    cur_res.append(cur.val)
                    queue = [cur.right,cur.left]+queue
            if cur_res:res.append(cur_res)
        return res

3. 剑指 Offer 32 - III. 从上到下打印二叉树 III


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class Solution:
    def levelOrder(self, root: TreeNode) -> List[List[int]]:
        queue=[root]
        res = []
        l = 1
        while(queue):
            cur_res=[]
            for i in range(len(queue)):
                cur = queue.pop()
                if cur:
                    if l%2:cur_res.append(cur.val)
                    else:cur_res=[cur.val]+cur_res
                    queue = [cur.right,cur.left]+queue
            l+=1
            if cur_res:res.append(cur_res)
        return res