Finding the common ancestor in a binary tree

Question

This question was asked to me in an interview: I have a binary tree and I have to find the common ancestor (parent) given two random nodes of that tree. I am also given a point

Answer 1

I think you could just do a search simultaneously for both nodes; the point at which the search diverges is the common ancestor.

commonAncestor tree a b:
  value := 
  if (a < value) && (b < value)
  then commonAncestor (left tree) a b
  else if (a > value) && (b > value)
  then commonAncestor (right tree) a b
  else tree

Interestingly this approach would scale to more than two nodes (check for all of them to be on the left side of tree, etc.)