练习来自Functional Swift (objc.io)。
轻微改动。

//🌲的声明
indirect enum Tree<T> {
    case Leaf
    case Node( Tree<T>,T,Tree<T> )
}
//计算有效数据个数
func count<T>(tree: Tree<T>) -> Int {
    switch tree {
    case .Leaf:
        return 0
    case let .Node(left, _, right):
        return 1 + count(left) + count(right)
    }
}
//空🌲
func emptySet<T>() -> Tree<T> {
    return Tree.Leaf
}
//检测是否为空🌲
func isEmptySet<T>(tree: Tree<T>) -> Bool {
    switch tree {
    case .Leaf:
        return true
    case .Node(_, _, _):
        return false
    }
}
//🌲中所有有效数据集合
func elements<T>(tree: Tree<T>) -> [T] {
    switch tree {
    case .Leaf:
        return []
    case let .Node(left, x, right):
        return elements(left) + [x] + elements(right)
    }
}
//检测是否为2叉搜索🌲
func isBST<T: Comparable>(tree: Tree<T>) -> Bool {
    switch tree {
    case .Leaf:
        return true
    case let .Node(left, x, right):
        return elements(left).reduce(true){ $1 < x }
            && elements(right).reduce(true){ $1 > x }
            && isBST(left)
            && isBST(right)
    }
}
//是否包含.（此方法低效）
func setContains<T: Comparable>(value: T, tree: Tree<T>) -> Bool {
    switch tree {
    case .Leaf:
        return false
    case let .Node(left, x, right):
        if value == x {
            return true
        }else if value < x {
            return setContains(value, tree: left)
        }else{
            return setContains(value, tree: right)
        }
    }
}
//插入
func setInsert<T: Comparable>(value: T, tree: Tree<T>) -> Tree<T> {
    switch tree {
    case .Leaf:
        return Tree.Node(.Leaf, value, .Leaf)
    case let .Node(left, x, right):
        if value == x {
            return tree
        }else if value < x {
            return Tree.Node(setInsert(value, tree: left), value, right)
        }else{
            return Tree.Node(left, value, setInsert(value, tree: right))
        }
    }
}