Most Prolific Level

1def mostProlificLevel(root: Optional[TreeNode]) -> int:
2    if not root:
3        return -1
4    from collections import deque, defaultdict
5    q = deque([(root, 0)])
6    level_count = defaultdict(int)
7    while q:
8        node, depth = q.popleft()
9        level_count[depth] += 1
10        if node.left:
11            q.append((node.left, depth + 1))
12        if node.right:
13            q.append((node.right, depth + 1))
14    best_level = 0
15    best = 0
16    for level in level_count:
17        if level + 1 in level_count and level_count[level + 1] > best:
18            best = level_count[level + 1]
19            best_level = level
20    return best_level