力扣（2024.08.07）

1. 637：二叉树的层平均值

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def averageOfLevels(self, root: Optional[TreeNode]) -> List[float]:
        def dfs(node, level, res):
            if not node:
                return
            if len(res) == level:
                res.append([])
            res[level].append(node.val)
            dfs(node.left, level + 1, res)
            dfs(node.right, level + 1, res)

        res = []
        level = 0
        dfs(root, level, res)

        final_res = []
        for i in res:
            final_res.append(sum(i) / len(i))
        return final_res

2. 429：N叉树的层序遍历

"""
# Definition for a Node.
class Node:
    def __init__(self, val=None, children=None):
        self.val = val
        self.children = children
"""

class Solution:
    def levelOrder(self, root: 'Node') -> List[List[int]]:
        def dfs(node, level, res):
            if not node:
                return
            if len(res) == level:
                res.append([])
            res[level].append(node.val)
            for child in node.children:
                dfs(child, level + 1, res)

        res = []
        level = 0
        dfs(root, level, res)
        return res       

3. 515：在每个树行中找最大值

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def largestValues(self, root: Optional[TreeNode]) -> List[int]:
        def dfs(node, level, res):
            if not node:
                return
            if len(res) == level:
                res.append([])
            res[level].append(node.val)
            dfs(node.left, level + 1, res)
            dfs(node.right, level + 1, res)

        res = []
        level = 0
        dfs(root, level, res)

        final_res = []
        for i in res:
            final_res.append(max(i))
        return final_res

4. 104：二叉树的最大深度

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def maxDepth(self, root: Optional[TreeNode]) -> int:
        def dfs(node, level, res):
            if not node:
                return
            if len(res) == level:
                res.append([])
            res[level].append(node.val)
            dfs(node.left, level + 1, res)
            dfs(node.right, level + 1, res)

        res = []
        level = 0
        dfs(root, level, res)
        return len(res)

5. 226：翻转二叉树

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def invertTree(self, root: Optional[TreeNode]) -> Optional[TreeNode]:
        def dfs(node):
            if not node:
                return
            node.left, node.right = node.right, node.left
            dfs(node.left)
            dfs(node.right)

        dfs(root)
        return root