swift递归检测对称二叉树

var isS: Bool = truefunc isSymmetric(_ root: TreeNode?) -> Bool {    helper(root, root)    return isS}func helper(_ left: TreeNode?, _ right: TreeNode?) {    if isS == false {        return    }    if left == nil {        isS = right == nil        return    }    if right == nil {        isS = left == nil        return    }    if left!.val != right!.val {        isS = false        return    }        let nodei = left?.left    let nodej = left?.right    let nodex = right?.left    let nodey = right?.right    helper(nodei, nodey)    helper(nodej, nodex)}