Binary tree longest consecutive sequence

Time: O(N); Space: O(H); medium

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).

Example 1:

Input: root = {TreeNode}

1
 \
  3
 / \
2   4
     \
      5

Output: 3

Explanation:

  • Longest consecutive sequence path is 3-4-5, so return 3.

Example 2:

Input: root = {TreeNode}

  2
   \
    3
   /
  2
 /
1

Output: 2

Explanation:

  • Longest consecutive sequence path is 2-3,not 3-2-1, so return 2.

[3]:

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

[4]:

class Solution1(object):
    """
    Time: O(N)
    Space: O(H)
    """
    def longestConsecutive(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        self.max_len = 0

        def longestConsecutiveHelper(root):
            if not root:
                return 0

            left_len = longestConsecutiveHelper(root.left)
            right_len = longestConsecutiveHelper(root.right)

            cur_len = 1
            if root.left and root.left.val == root.val + 1:
                cur_len = max(cur_len, left_len + 1)
            if root.right and root.right.val == root.val + 1:
                cur_len = max(cur_len, right_len + 1)

            self.max_len = max(self.max_len, cur_len)

            return cur_len

        longestConsecutiveHelper(root)

        return self.max_len

[5]:

s = Solution1()
root = TreeNode(1)
root.right = TreeNode(3)
root.right.left = TreeNode(2)
root.right.right = TreeNode(4)
root.right.right.right = TreeNode(5)
assert s.longestConsecutive(root) == 3

root = TreeNode(2)
root.right = TreeNode(3)
root.right.left = TreeNode(2)
root.right.left.left = TreeNode(1)
assert s.longestConsecutive(root) == 2