[Python] maximum depth of a binary tree using BFS- why is the depth doubled what it is...

I am writing iterative solution to finding the max depth of a binary tree- using Breadth First Search (BFS):

class TreeNode:
def __init__(self, val=0, left=None, right=None):
self.val = val
self.left = left
self.right = right

from collections import deque
class Solution:
def maxDepth(self, root: TreeNode) -> int:
if not root:
return 0

queue = deque()
queue.append(root)
depth = 1
level_length = len(queue)

while queue:
for i in range(level_length):
node = queue.popleft()

if node.left:
queue.append(node.left)

if node.right:
queue.append(node.right)

depth +=1

return depth


For the input root = [3,9,20,null,null,15,7], I am getting output=6 rather than 3, which is clearly the correct answer.

I've gone through my code several times on paper, but can't seem to see where it is falling short- any ideas?

Continue reading...