226. Invert Binary Tree

Description:

Given the root of a binary tree, invert the tree, and return its root.

# 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

'''
[1] recursive - top down)

idea::
recursively invert left and right

code::
    def invertTree(self, root: Optional[TreeNode]) -> Optional[TreeNode]:
        self.invert(root)
        return root
        
    def invert(self,root):
        if root is None:
            return
        root.left,root.right = root.right,root.left
        
        self.invert(root.left)
        self.invert(root.right)
        
-T/C: O(n) # n==the number of TreeNode
-S/C: O(h) # h==height of Tree
'''

'''
[2] iterative)

idea::
recursive solution can be converted to iterative solution using stack

code::
    def invertTree(self, root: Optional[TreeNode]) -> Optional[TreeNode]:
        stack=[root]
        
        while stack:
            cur=stack.pop()
            
            # if cur is not None
            if cur:
                stack.append(cur.left)
                stack.append(cur.right)
                # swap
                cur.left, cur.right = cur.right, cur.left
        
        return root
        
-T/C: O(n)
-S/C: O(h)
'''
class Solution:
    def invertTree(self, root: Optional[TreeNode]) -> Optional[TreeNode]:
        stack=[root]
        
        while stack:
            cur=stack.pop()
            
            # if cur is not None
            if cur:
                stack.append(cur.left)
                stack.append(cur.right)
                # swap
                cur.left, cur.right = cur.right, cur.left
        
        return root