func mergeBinaryTrees(node1: TreeNode?, node2: TreeNode?) -> TreeNode? {

    if (node1 == null) return node2
    if (node2 == null) return node1

    var stack1 = [TreeNode?]()
    var stack2 = [TreeNode?]()

    stack1.append(node1)
    stack2.append(node2)

    while !stack1.isEmpty || !stack2.isEmpty {
        let curr1: TreeNode! = stack1.popLast()!
        let curr2: TreeNode! = stack2.popLast()!
        let dummy = curr1
        dummy?.value = curr1.value + curr2.value

        if curr1.left == nil || curr2.left == nil {
            dummy?.left = curr1.left == nil ? curr2.left : curr1.left
        } else if curr1.left != nil && curr2.left != nil {
            stack1.append(curr1.left)
            stack2.append(curr2.left)
        }

        if curr1.right == nil || curr2.right == nil {
            dummy?.right = curr1.right == nil ? curr2.right : curr1.right
        } else if curr1.right != nil && curr2.right != nil {
            stack1.append(curr1.right)
            stack2.append(curr2.right)
        }
    }
    return node1
}

let node4 = TreeNode(value: 4, left: nil, right: nil)
let node5 = TreeNode(value: 5, left: nil, right: nil)
let node6 = TreeNode(value: 6, left: nil, right: nil)
let node2 = TreeNode(value: 2, left: node4, right: node5)
let node3 = TreeNode(value: 3, left: nil, right: node6)
let node1 = TreeNode(value: 1, left: node2, right: node3)

let node3b = TreeNode(value: 3, left: nil, right: nil)
let node2b = TreeNode(value: 2, left: nil, right: nil)
let node6b = TreeNode(value: 6, left: nil, right: nil)
let node1b = TreeNode(value: 1, left: node3b, right: nil)
let node7 = TreeNode(value: 7, left: node2b, right: node6b)
let node4b = TreeNode(value: 4, left: node1b, right: node7)

let node = mergeBinaryTrees(node1: node1, node2: node4b)

func inorder(node: TreeNode?) {
    if node  == nil {
        return
    }

    inorder(node: node?.left)
    print(node?.value)
    inorder(node: node?.right)
}

inorder(node: node)