swift实现一个函数，用来判断一颗二叉树是不是对称的。注意，如

class TreeNode {

var previous: TreeNode?

var next: TreeNode?

var data: Int?

func isSymmeTrical(node: TreeNode?) -> Bool {
    if node == nil || (node?.previous == nil && node?.next == nil) {
        return true
    }
    
    return isSymmeTrical(left: node!.previous, right: node!.next)
}

private func isSymmeTrical(left: TreeNode?, right: TreeNode?) -> Bool {
    
    if left == nil && right == nil {
        return true
    } else if left == nil || right == nil {
        return false
    } else if left?.data != right?.data {
        return false
    }
    return isSymmeTrical(left: left?.previous, right: left?.next) && isSymmeTrical(left: right?.previous, right: right?.next)
}

}