Sample Tree

class Tree : NSObject {
    var nodes:[Node] = [Node]()

    func output(){
        let parents = nodes.filter{ $0.isParent }

        parents.first?.output()
    }

    func addRelationship(_ rel:String) {
        var ids:[String] = rel.components(separatedBy: ",")

        let parentNode = getNode(withID: ids[0])
        parentNode.isParent = true
        for i in 1 ..< ids.count{
            let child:Node = getNode(withID: ids[i])
            child.isParent = false
            parentNode.children.append(child)

        }
    }

    func getNode( withID id:String ) -> Node{
        let ref : [Node] = nodes.filter{ id == $0.id }
        if ref.count > 0 {
            return ref.first!
        }
        let node = Node(withId: id)
        nodes.append(node)
        return node
    }
}

class Node:NSObject {
    let id:String
    var isParent:Bool = false
    var children:[Node] = [Node]()

    init ( withId id:String ){
        self.id = id
    }

    func output(order : Int = 0) {
        var desc = (0...order).map{_ in "   "}.joined() + id
        print(desc)
        for i in children {
            i.output(order: order + 1)
        }
    }
}

let input:[String] = ["B2,E5,F6", "A1,B2,C3,D4", "D4,G7,I9", "G7,H8"]
let organizations:Tree = Tree()

_ = input.map{ organizations.addRelationship($0) }

organizations.output()