longest_consecutive.py

"""
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
"""


def longest_consecutive(root):
    """
    :type root: TreeNode
    :rtype: int
    """
    if not root:
        return 0
    maxlen = 0
    dfs(root, 0, root.val, maxlen)
    return maxlen


def dfs(root, cur, target, maxlen):
    if not root:
        return
    if root.val == target:
        cur += 1
    else:
        cur = 1
    maxlen = max(cur, maxlen)
    dfs(root.left, cur, root.val+1, maxlen)
    dfs(root.right, cur, root.val+1, maxlen)