32、从上到下打印二叉树

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。

例如:
给定二叉树: [3,9,20,null,null,15,7]

返回：[3,9,20,15,7]

=============队列=============

# 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]:
        if not root:
            return []
        from collections import deque
        q = deque()
        q.append(root)
        res = []
        while q:
            cur = q.popleft()
            res.append(cur.val)
            if cur.left:
                q.append(cur.left)
            if cur.right:
                q.append(cur.right)
        return res

从上到下按层打印二叉树，同一层的节点按从左到右的顺序打印，每一层打印到一行。

例如:
给定二叉树: [3,9,20,null,null,15,7],

返回其层次遍历结果：

[
  [3],
  [9,20],
  [15,7]
]

# 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[List[int]]:
        if not root:
            return []
        from collections import deque
        q = deque()
        q.append(root)
        ans = []
        while q:
            tmp = deque()
            res = []
            while q:
                cur = q.popleft()
                res.append(cur.val)
                if cur.left:
                    tmp.append(cur.left)
                if cur.right:
                    tmp.append(cur.right)
            q = tmp
            ans.append(res)
        return ans

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

例如:
给定二叉树: [3,9,20,null,null,15,7],

返回其层次遍历结果：

[
  [3],
  [20,9],
  [15,7]
]

# 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[List[int]]:
        if not root:
            return []
        from collections import deque
        q = deque()
        q.append(root)
        ans = []
        flag = 1
        while q:
            tmp = deque()
            res = []
            while q:
                cur = q.popleft()
                res.append(cur.val)
                if cur.left:
                    tmp.append(cur.left)
                if cur.right:
                    tmp.append(cur.right)
            q = tmp
            if flag == 1:
                ans.append(res)
            else:
                ans.append(res[::-1])
            flag = -flag

        return ans