A strobogrammatic number is a number that looks the same when rotated 180 degrees (looked at upside down).

Write a function to determine if a number is strobogrammatic. The number is represented as a string.

Example 1:

Input:  "69"
Output: true

Example 2:

Input:  "88"
Output: true

Example 3:

Input:  "962"
Output: false

strobogramatic數字是旋轉180度時看起來相同的數字（上下顛倒）。

寫一個函式來確定一個數字是否是頻閃的。數字用字串表示。

例1：

輸入：“69”

輸出：true

例2：

輸入：“88”

輸出：真

例3：

輸入：“962”

輸出：false

 1 class Solution {
 2     func isStrobogrammatic(_ num:String) -> Bool {
 3         var m:[Character:Character] = ["0":"
 
0","1":"1","8":"8","6":"9","9":"6"]
 4         let len = num.count
 5         for i in 0...len/2
 6         {
 7             if m[num[i]] != num[len - i - 1]
 8             {
 9                 return false
10             }
11         }
12         return true
13     }
14 }
15 
16 extension String {        
 
17     //subscript函式可以檢索陣列中的值
18     //直接按照索引方式擷取指定索引的字元
19     subscript (_ i: Int) -> Character {
20         //讀取字元
21         get {return self[index(startIndex, offsetBy: i)]}
22     }
23 }