[Swift]LeetCode246.對稱數 $ Strobogrammatic Number

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?微信公眾號：山青詠芝（shanqingyongzhi）
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?GitHub地址：https://github.com/strengthen/LeetCode
?原文地址：https://www.cnblogs.com/strengthen/p/10214231.html?
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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）。

寫一個函數來確定一個數字是否是頻閃的。數字用字符串表示。

例1：

輸入：“69”

輸出：true

例2：

輸入：“88”

輸出：真

例3：

輸入：“962”

輸出：false

---

class Solution {
    func isStrobogrammatic(_ num:String) -> Bool {
        var m:[Character:Character] = ["0":"0","1":"1","8":"8","6":"9","9":"6"]
        let len = num.count
        for i in 0...len/2
        {
            if m[num[i]] != num[len - i - 1]
            {
                return false
            }
        }
        return true
    }
}

extension String {        
    //subscript函數可以檢索數組中的值
    //直接按照索引方式截取指定索引的字符
    subscript (_ i: Int) -> Character {
        //讀取字符
        get {return self[index(startIndex, offsetBy: i)]}
    }
}

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