func mergeTrees(node1: TreeNode?, node2: TreeNode?) -> TreeNode? {
    if node1 == nil {
        return node2
    }

    if node2 != nil {
        node1!.value += node2!.value
        node1?.left = mergeTrees(node1: node1?.left, node2: node2?.left)
        node1?.right = mergeTrees(node1: node1?.right, node2: node2?.right)
    }
    return node1
}