func lowestCommonAncestor(root, p, q *TreeNode) *TreeNode {
     if root == p || root == q || root == nil {
         return root
     }
     
     left := lowestCommonAncestor(root.Left, p, q)
     right := lowestCommonAncestor(root.Right, p, q)
     
     if left == nil && right == nil {
         return nil
     }
     
     if left != nil && right != nil {
         return root
     }
     
     if (left != nil) {
         return left
     }
     
     return right
}