114. Flatten Binary Tree to Linked List

func flatten(_ root: TreeNode?) {
        guard let root = root else {
            return
        }
        flattenHelper(root)
    }


    func flattenHelper(_ node : TreeNode) -> (TreeNode, TreeNode){
        if node.left == nil && node.right == nil {
             return (node, node)
        }
        if node.left == nil {
            let rigthtRes = flattenHelper(node.right!)
            return (node, rigthtRes.1)
        }
        if node.right == nil {
            let leftRes = flattenHelper(node.left!)
            node.left = nil
            node.right = leftRes.0
            return (node, leftRes.1)
        }

        let leftRes = flattenHelper(node.left!)
        let rigthtRes = flattenHelper(node.right!)
        node.left = nil
        node.right = leftRes.0
        leftRes.1.right = rigthtRes.0
        return (node, rigthtRes.1)

    }