I gave the hints to help the interviewee to learn the algorithm

longest univalue path - hints to help

Leetcode: longest univalue path
hint:

instead of working on longest path directly, kind of greedy, break into two parts. 

length1 = one is the root to leaf node longest path -> left side
length2 = one is the root to leaf node longest path -> right side

path cross the root = length1 + length2

In order to calculate length1 or length2, recursive can be used to calculate. Brute force all paths
from the tree, any node to any node, n nodes in the tree, n * (n-1) = O(n*n). It is hard to compute. 

Instead brute force every node as root node of the tree, determine the longest path cross the root node. 

More hint:

work on those simple trees:
4       4       4         4
       /       / \       / \
      4       4   4     4   4
                       /
                      4
 Recursive function:
 return 0, 1, 1, 2
 longest path:
 return 0, 1, 2, 3