[Swift]LeetCode248.对称数 III $ Strobogrammatic Number III

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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 count the total strobogrammatic numbers that exist in the range of low <= num <= high.

Example:

Input: low = "50", high = "100"
Output: 3 
Explanation: 69, 88, and 96 are three strobogrammatic numbers.

Note:
Because the range might be a large number, the lowand high numbers are represented as string.

---

strobogramatic数字是旋转180度时看起来相同的数字（上下颠倒）。

编写一个函数来计算在low<=num<=high范围内存在的总strobogrammatic数。

例子：

输入：low=“50”，high=“100”

输出：3

说明：69、88和96是三个strobogramatic数字。

注：

因为范围可能是一个大数字，所以低数字和高数字表示为字符串。

---

class Solution {
    func strobogrammaticInRange(_ low:String,_ high:String) -> Int {
        var res:Int = 0
        find(low, high, "",  &res)
        find(low, high, "0", &res)
        find(low, high, "1", &res)
        find(low, high, "8", &res)
        return res;
    }
    
    func find (_ low:String,_ high:String,_ w:String,_ res:inout Int)
    {
        if w.count >= low.count && w.count <= high.count
        {
            if (w.count == low.count && w < low) || (w.count == high.count && w > high)
            {
                return 
            }
            if !(w.count > 1 && w[0] == "0")
            {
                res += 1
            }
        }
        if w.count + 2 > high.count
        {
            return
        }
        find(low, high, "0" + w + "0", &res)
        find(low, high, "1" + w + "1", &res)
        find(low, high, "6" + w + "9", &res)
        find(low, high, "8" + w + "8", &res)
        find(low, high, "9" + w + "6", &res)
    }
}

extension String {        
    //subscript函数可以检索数组中的值
    //直接按照索引方式截取指定索引的字符
    subscript (_ i: Int) -> Character {
        //读取字符
        get {return self[index(startIndex, offsetBy: i)]}
    }
}