My solution was so convoluted.

vector pathTo(node* root,int key){
  vector path;
  node* current = root;

  while (current != NULL && current->data != key) {
    path.push_back(current);
    if      (key data) current = current->left;
    else if (key > current->data) current = current->right;

  }
  if (current)
    path.push_back(current);
  return path;
}

node*lca(node*root,int x,int y){
  vector pathX = pathTo(root, x),
                pathY = pathTo(root, y);

  unordered_set s;
  int i;
  for (i = pathX.size()-1; i >= 0; --i)
    s.insert(pathX[i]);
  for (i = pathY.size()-1; i >= 0; --i) {
    if (!s.insert(pathY[i]).second) break;
  }
  return pathY[i];
}