Level Order Traversal BFS -- Simple to understand!!

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    def addOneRow(self, root, v, d):
        # special case where depth is one
        if d == 1:
            new_root = TreeNode(v)
            new_root.left = root
            return new_root
        queue = collections.deque([root])
        level = 1
        while queue:
            size = len(queue)
            # level order traversal
            for i in range(size):
                curr = queue.popleft()
                # insert nodes if we reach 1 level before d
                if level == d-1:
                    old_left, old_right = curr.left, curr.right
                    curr.left, curr.right = TreeNode(v), TreeNode(v)
                    curr.left.left, curr.right.right = old_left, old_right
                    if curr.left: 
                    if curr.right:
            # return root once we've appended
            if level == d-1:
                return root
            level += 1
        return None

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