[Swift]LeetCode828. 獨特字符串 | Unique Letter String

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A character is unique in string S if it occurs exactly once in it.

For example, in string S = "LETTER", the only unique characters are "L" and "R".

Let's define UNIQ(S) as the number of unique characters in string S.

For example, UNIQ("LETTER") =  2.

Given a string S with only uppercases, calculate the sum of UNIQ(substring) over all non-empty substrings of S.

If there are two or more equal substrings at different positions in S, we consider them different.

Since the answer can be very large, return the answer modulo 10 ^ 9 + 7.

Example 1:

Input: "ABC"
Output: 10
Explanation: All possible substrings are: "A","B","C","AB","BC" and "ABC".
Evey substring is composed with only unique letters.
Sum of lengths of all substring is 1 + 1 + 1 + 2 + 2 + 3 = 10

Example 2:

Input: "ABA"
Output: 8
Explanation: The same as example 1, except uni("ABA") = 1.

Note: 0 <= S.length <= 10000.

---

若是一個字符在字符串 S 中有且僅有出現一次，那麼咱們稱其爲獨特字符。

例如，在字符串 S = "LETTER" 中，"L" 和 "R" 能夠被稱爲獨特字符。

咱們再定義 UNIQ(S) 做爲字符串 S 中獨特字符的個數。

那麼，在 S = "LETTER" 中， UNIQ("LETTER") =  2。

對於給定字符串 S，計算其全部非空子串的獨特字符的個數，即 UNIQ(substring)。

若是出現兩個或者多個相同的子串，將其認爲是不一樣的兩個子串。

考慮到答案可能會很是大，規定返回格式爲：結果 mod 10 ^ 9 + 7。

示例 1:

輸入: "ABC"
輸出: 10
解釋: 全部可能的子串爲："A","B","C","AB","BC" 和 "ABC"。
     其中，每個子串都由獨特字符構成。
     因此其長度總和爲：1 + 1 + 1 + 2 + 2 + 3 = 10

示例 2:

輸入: "ABA"
輸出: 8
解釋: 除了子串 UNIQ('ABA') = 1，其他與示例1相同。

說明: 0 <= S.length <= 10000。

---

Runtime: 4344 ms

Memory Usage: 19.8 MB

 1 class Solution {
 2     func uniqueLetterString(_ S: String) -> Int {
 3         var index:[[Int]] = [[Int]](repeating:[Int](repeating:-1,count:2),count:26)
 4         var res:Int = 0
 5         var N:Int = S.count
 6         var mod:Int = Int(pow(10.0,9.0)) + 7
 7         for i in 0..<N
 8         {
 9             var c:Int = S[i].ascii - 65
10             res = (res + (i - index[c][1]) * (index[c][1] - index[c][0]) % mod) % mod
11             index[c] = [index[c][1], i]
12         }
13         for c in 0..<26
14         {
15             res = (res + (N - index[c][1]) * (index[c][1] - index[c][0]) % mod) % mod
16         }
17         return res
18     }
19 }
20 
21 //String擴展
22 extension String {        
23     //subscript函數能夠檢索數組中的值
24     //直接按照索引方式截取指定索引的字符
25     subscript (_ i: Int) -> Character {
26         //讀取字符
27         get {return self[index(startIndex, offsetBy: i)]}
28     }
29 }
30 
31 //Character擴展 
32 extension Character  
33 {  
34   //Character轉ASCII整數值(定義小寫爲整數值)
35    var ascii: Int {
36        get {
37            return Int(self.unicodeScalars.first?.value ?? 0)
38        }       
39     }
40 }