CodecII.swift

class Codec {
    func serializeHelper(_ root: TreeNode?,_ output: inout String) {
        if root == nil { 
            output = output + "null" + " "
            return 
        }
        output = output + String(root!.val) + " "
        
        serializeHelper(root?.left, &output)
        serializeHelper(root?.right, &output)
    }
    func serialize(_ root: TreeNode?) -> String {
        if root == nil { return "" }
        var output = ""
        serializeHelper(root, &output)
        print(output)
        return output
    }
    func deserializehelper(_ result: inout [String]) -> TreeNode? {
        if result.isEmpty == true { return nil }
        let value = result.removeFirst()
        var node: TreeNode?
        if value == "null" {
            return nil
        } else {
            node = TreeNode(Int(value)!)
        }
        node?.left = deserializehelper(&result)
        node?.right = deserializehelper(&result)
        return node
    }
    
    func deserialize(_ data: String) -> TreeNode? {
        if data.isEmpty { return nil }
        var result = data.components(separatedBy: " ")
        return deserializehelper(&result)
    }
}