TreeAnyGenerator.swift

extension Tree : SequenceType {
  public func generate() -> AnyGenerator<Element> {
    var stack: [Tree<Element>] = []
    var curr = self
    return anyGenerator {
      while case let .Node(_, l, _, _) = curr {
        stack.append(curr)
        curr = l
      }
      if case let .Node(_, _, x, r)? = stack.popLast() {
        curr = r
        return x
      }
      return nil
    }
  }
}

TreeGeneratorStruct.swift

public struct TreeGenerator<Element : Comparable> : GeneratorType {
  private var (stack, curr): ([Tree<Element>], Tree<Element>)
  public mutating func next() -> Element? {
    while case let .Node(_, l, x, r) = curr {
      if case .Empty = l {
        curr = r
        return x
      }
      stack.append(curr)
      curr = l
    }
    if case let .Node(_, _, x, r)? = stack.popLast() {
      curr = r
      return x
    }
    return nil
  }
}

extension Tree : SequenceType {
  func generate() -> TreeGenerator<Element> {
    return TreeGenerator(stack: [], curr: self)
  }
}