Kth Smallest Element in a BST

kthSmallest.swift

func kthSmallest(_ root: TreeNode?, _ k: Int) -> Int {
        guard let _ = root else {
            return -1
        }
        
        var count = 0
        var stack = [TreeNode]()
        var root = root
        
        while !stack.isEmpty || root != nil {
            while root != nil {
                stack.append(root!)
                root = root!.left
            }
            
            root = stack.removeLast()
            count += 1
            if count == k {
                break
            }
            root = root!.right
        }
        
        return root!.val
    }