enum Node {
 Branch(Tree),
 Leaf(String)
}

struct Tree {
  left: Vec<Node>,
  right: Vec<Node>
}

impl Tree {
  pub fn names(&self) -> Vec<&str> {
    let mut name_strs = Vec::new();
    for node in (&self.left).iter().chain((&self.right).iter()) {
        match node {
            Node::Branch(t) => name_strs.extend(t.names()),
            Node::Leaf(s) => name_strs.push(s)
        }
    }
    name_strs
  }
}

#[cfg(test)]
mod tests {
    use super::{Tree,Node};
    #[test]
    fn test_names() {
        let test_tree = Tree{
            left: vec![Node::Branch(Tree{left: vec![Node::Leaf("H".to_string()), Node::Leaf("e".to_string())], right: vec![Node::Leaf("ll".to_string()), Node::Leaf("o".to_string())]}),
                   Node::Branch(Tree{left: vec![Node::Leaf(", ".to_string()), Node::Leaf("W".to_string())], right: vec![]})],
            right: vec![Node::Branch(Tree{left: vec![Node::Leaf("orld!".to_string())], right: vec![]})]
        };
        assert_eq!(test_tree.names(), vec!["H", "e", "ll", "o", ", ", "W", "orld!"]);
    }

}