input: Write a python function to find the lowest common ancestor (LCA) of two nodes in a binary tree.
  assert find_lca(root, node1, node2) == expected_node
  assert find_lca(root, node3, node4) == expected_node
  assert find_lca(root, node5, node6) == expected_node
output: def find_lca(root, p, q):
      if root is None:
          return None
  
      if root == p or root == q:
          return root
  
      left_lca = find_lca(root.left, p, q)
      right_lca = find_lca(root.right, p, q)
  
      if left_lca and right_lca:
          return root
  
      return left_lca if left_lca is not None else right_lca