First Common Ancestor

Design an algorithm to find the first common ancestor of two nodes in a binary tree.

Avoid storing additional nodes in a data structure.

Note: This is not necessarily a binary search tree.

Example:

       8
      / \
     3   1
    / \
   2   4

FCA(2, 4) = 3
FCA(2, 3) = 3
FCA(2, 1) = 8