[Swift]LeetCode249.群组偏移字符串 $ Group Shifted Strings

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Given a string, we can "shift" each of its letter to its successive letter, for example: "abc" -> "bcd". We can keep "shifting" which forms the sequence:

"abc" -> "bcd" -> ... -> "xyz"

Given a list of strings which contains only lowercase alphabets, group all strings that belong to the same shifting sequence.

For example, given: ["abc", "bcd", "acef", "xyz", "az", "ba", "a", "z"], 
Return:

[
  ["abc","bcd","xyz"],
  ["az","ba"],
  ["acef"],
  ["a","z"]
]

Note: For the return value, each inner list's elements must follow the lexicographic order.

---

给定一个字符串，我们可以将它的每个字母“移位”到它的连续字母，例如：“abc”->“bcd”。我们可以保持“移动”，这形成了一个序列：

"abc" -> "bcd" -> ... -> "xyz"

给定只包含小写字母的字符串列表，将属于同一移位序列的所有字符串分组。

例如，给定：["abc", "bcd", "acef", "xyz", "az", "ba", "a", "z"], 

返回：

[
  ["abc","bcd","xyz"],
  ["az","ba"],
  ["acef"],
  ["a","z"]
]

注意：对于返回值，每个内部列表的元素必须遵循字典顺序。

---

class Solution {
    func groupStrings(_ strings:[String]) -> [[String]] {
        var res:[[String]] = [[String]]()
        var m:[String:Set<String>] = [String:Set<String>]()
        for a in strings
        {
            var t:String = ""
            for c in a.characters
            {
                t += String((c.ascii + 26 - a[0].ascii) % 26) + ","
            }
            if m[t] == nil
            {
              m[t] = Set<String>()
            }
             m[t]!.insert(a) 
        }
        for it in m.values
        {
            res.append(Array(it))
        }
        return res
    }
}

extension Character  
{  
  //属性：ASCII整数值(定义小写为整数值)
   var ascii: Int {
        get {
            let s = String(self).unicodeScalars
            return Int(s[s.startIndex].value)
        }
    }
}

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)]}
    }
}