Binary Tree Longest Consecutive Sequence

lcpy gen -n 298   # by problem number
lcpy gen -s binary_tree_longest_consecutive_sequence   # by problem name

Input: root = [1,null,3,2,4,null,null,null,5]
Output: 3
Explanation: Longest consecutive sequence path is 3-4-5, so return 3.

Input: root = [2,null,3,2,null,1]
Output: 2
Explanation: Longest consecutive sequence path is 2-3, not 3-2-1, so return 2.

from leetcode_py import TreeNode


class Solution:
    # Time: O(n) — each node visited once
    # Space: O(h) — recursion depth equals tree height
    def longest_consecutive(self, root: TreeNode[int] | None) -> int:
        def dfs(node: TreeNode[int] | None, parent_val: int | None, length: int) -> int:
            if node is None:
                return length
            length = length + 1 if parent_val is not None and node.val - parent_val == 1 else 1
            return max(
                length,
                dfs(node.left, node.val, length),
                dfs(node.right, node.val, length),
            )

        return dfs(root, None, 0)