TreeInOrder.swift

public class TreeNode {
    public let val: Int
    public var left: TreeNode?
    public var right: TreeNode?
    init(_ val: Int) {
        self.val = val
    }
}

func inorder(_ root: TreeNode) -> [Int] {
    var stack = [TreeNode]()
    var result = [Int]()
    var current: TreeNode? = root
    while current != nil || !stack.isEmpty {
        if let currentNode = current {
            stack.append(currentNode)
            current = currentNode.left
        } else {
            let popped = stack.removeLast()
            result.append(popped.val)
            current = popped.right
        }
    }
    return result
}