题目：https://www.nowcoder.com/ta/coding-interviews?page=3

请实现一个函数按照之字形打印二叉树，即第一行按照从左到右的顺序打印，第二层按照从右至左的顺序打印，第三行按照从左到右的顺序打印，其他行以此类推。

考虑：跟之前按行打印二叉树的思路，先将二叉树按行分别存入二维列表，然后将特定的子列表翻转

列表翻转：

1.https://www.runoob.com/python3/python-reversing-list.html

2.https://www.runoob.com/python3/python-slicing-rotate-string.html

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#列表翻转
a=[1,2,3,4,5,6,7,8,9] 
print(a) 
d=a[::-1]#对列表切片翻转
print(a) 
c=a
print(d) 
c.reverse() #会改变原有列表
print(c) 
print(a) #??为什么a会被改变
[1, 2, 3, 4, 5, 6, 7, 8, 9]
[1, 2, 3, 4, 5, 6, 7, 8, 9]
[9, 8, 7, 6, 5, 4, 3, 2, 1]
[9, 8, 7, 6, 5, 4, 3, 2, 1]
[9, 8, 7, 6, 5, 4, 3, 2, 1]
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def Reverse(lst): 
    lst.reverse() 
    return lst 
      
a=[1,2,3,4,5,6,7,8,9] 
print(Reverse(a))
[9, 8, 7, 6, 5, 4, 3, 2, 1]

for循环步长：https://www.runoob.com/python3/python3-loop.html

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#将索引为奇数（第偶数：2、4）个子列表翻转
res = [[8], [6, 10], [5, 7, 9, 11], [12, 13]]
for level in range(1, len(res), 2):
    res[level] = res[level][::-1]
print(res)
[[8], [10, 6], [5, 7, 9, 11], [13, 12]]
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class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None
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root = TreeNode(8)
a = TreeNode(6)
b = TreeNode(10)
c = TreeNode(5)
d = TreeNode(7)
e = TreeNode(9)
f = TreeNode(11)
g = TreeNode(12)
h = TreeNode(13)
 
root.left = a
root.right = b
a.left = c
a.right = d
b.left = e
b.right = f
d.left = g
d.right = h
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class Solution:
    def Print(self, pRoot):
        if not pRoot:
            return []
        result = []
        level = 0
        self.level(pRoot, level, result)#和之前一样，得到按行对应的二维列表
        
        for level in range(1, len(result), 2):#将索引为奇数（第偶数：2、4）个子列表翻转
            result[level] = result[level][::-1]
        return result
    
    def level(self, root, level, result):#和之前一样，得到按行对应的二维列表
        if not root:
            return None
        if level == len(result):
            result.append([])
        result[level].append(root.val)
        if root.left:
            self.level(root.left, level+1, result)
        if root.right:
            self.level(root.right, level+1, result)
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method = Solution()
answer = method.Print(root)
answer
[[8], [10, 6], [5, 7, 9, 11], [13, 12]]

参考：https://blog.csdn.net/fuxuemingzhu/article/details/79724959