## 題解

### DFS

# 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 not root:
            return root
        self.invertTree(root.left)
        self.invertTree(root.right)
        root.left, root.right = root.right,root.left
        return root

### BFS

# 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]:
        queue = [root]

        while queue:
            length = len(queue)
            for i in range(length):
                node = queue.pop(0)
                if node:
                    if node.left:
                        queue.append(node.left)
                    if node.right:
                        queue.append(node.right)
                    node.left, node.right = node.right, node.left

        return root