Inorder - NON- RECURSION

var stack = [TreeNode]()
    func inorderTraversal(_ root: TreeNode?) -> [Int] {
        var res = [Int]()
        pushLeft(root)

        while stack.count > 0{
            let cur = stack.removeLast()
            res.append(cur.val)
            pushLeft(cur.right)
        }
        return res

    }
    func pushLeft(_ node : TreeNode?){
        var runner = node
        while runner != nil{
            stack.append(runner!)
            runner = runner!.left
        }
    }