Binary tree - Mirror/Invert

"""
https://leetcode.com/problems/invert-binary-tree/description/

Mirror/Invert a binary tree

The way to solve this is to think, "Ok, I just need to do a BFS/DFS on the tree and swap the left and right as I go." So, you need to already know how to do a BFS/DFS.
"""

# Recursive, using Vertex objects

class V:  # Vertex
    def __init__(self, value=0, left=None, right=None):
        self.value = value
        self.left = left
        self.right = right

def invert(root):
    if root is None:
        return
    root.left, root.right = root.right, root.left
    invert(root.left)
    invert(root.right)


root = V(1)
root.left = V(2, V(4), V(5))
root.right = V(3, V(6), V(7))

assert(root.left.left.value == 4)
invert(root)
assert(root.left.left.value == 7)



###### Iterative implementation:

def invert(graph, head):
    stack = [head]

    while stack:
        curr = stack.pop()
        if curr is None:
            continue
        curr.left, curr.right = curr.right, curr.left
        stack.extend([curr.left, curr.right])