LC226. Invert Binary Tree

# 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 None
        
        if not root.left and not root.right:
            return root
        
        root.left, root.right = root.right, root.left
 
        self.invertTree(root.left)
        self.invertTree(root.right)
 
        return root

Complexity

Time: $O(n)$
Space: $O(n)$ for function stack