226 反转二叉树

题目226反转二叉树

思路

方法一：前序

方法二：后序

方法三：中序
注意：画画图就知道为什么都是在处理左子树了。

代码

# 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]:
        if root is None:
            return None
        # 方法一：前序
        # temp = root.left
        # root.left = root.right
        # root.right = temp
        # self.invertTree(root.left)
        # self.invertTree(root.right)
        # return root
        # 方法二：后序
        # self.invertTree(root.left)
        # self.invertTree(root.right)
        # temp = root.left
        # root.left = root.right
        # root.right = temp
        # return root
		# 中序
        self.invertTree(root.left)
        temp = root.left
        root.left = root.right
        root.right = temp
        self.invertTree(root.left)
        return root