Leetcode: Binary Tree Longest Consecutive Sequence

Question

Given a binary tree, find the length of the longest consecutive sequence path.

The path refers to any sequence of nodes from some starting node to any node in the tree along the parent-child connections. The longest consecutive path need to be from parent to child (cannot be the reverse).

For example,
1
\
3
/ \
2 4
\
5
Longest consecutive sequence path is 3-4-5, so return 3.
2
\
3
/
2
/
1
Longest consecutive sequence path is 2-3,not3-2-1, so return 2.
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My solution

# Definition for a binary tree node.
# class TreeNode(object):
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None

class Solution(object):
    def longestConsecutive(self, root):
        """ :type root: TreeNode :rtype: int """

        self.length = 0
        self.helper(root)
        return self.length

    def helper(self, root):
        if root==None:
            return -1

        l = self.helper(root.left)
        r = self.helper(root.right)

        left  = True if root.left!=None and root.val == root.left.val -1 else False
        right = True if root.right!=None and root.val == root.right.val -1 else False

        if left and right:
            self.length = max( [self.length, 1+l, 1+r] )
            return max([1+l, 1+r])
        if left and not right:
            self.length = max( [self.length, 1+l] )
            return 1+l
        if not left and right:
            self.length = max( [self.length, 1+r] )
            return 1+r
        else:
            self.length = max( [self.length, 1] )
            return 1