inorder Successor

func inorderSuccessor(_ root: TreeNode?, _ p: TreeNode?) -> TreeNode? {
        guard let root = root else {return nil}
        guard let p = p else {return nil}

        var runner = root
        var stack = [TreeNode]()

        stack.append(root)
        while runner.left != nil{
            stack.append(runner.left!)
            runner = runner.left!
        }

        while stack.count > 0{
            let cur = stack.removeLast()
            if cur.right != nil{
                stack.append(cur.right!)
                runner = cur.right!
                while runner.left != nil{
                    stack.append(runner.left!)
                    runner = runner.left!
                }
            }
            //检查当前的如果就是P, 那么下一轮的那个node就是successor
            if cur === p {
                return stack.removeLast()
            }
        }
        return nil
    }