[Swift]LeetCode940. 不一樣的子序列 II | Distinct Subsequences II

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Given a string S, count the number of distinct, non-empty subsequences of S .

Since the result may be large, return the answer modulo 10^9 + 7.

Example 1:

Input: "abc"
Output: 7 Explanation: The 7 distinct subsequences are "a", "b", "c", "ab", "ac", "bc", and "abc". 

Example 2:

Input: "aba"
Output: 6 Explanation: The 6 distinct subsequences are "a", "b", "ab", "ba", "aa" and "aba". 

Example 3:

Input: "aaa"
Output: 3 Explanation: The 3 distinct subsequences are "a", "aa" and "aaa".

Note:

1. S contains only lowercase letters.
2. 1 <= S.length <= 2000

---

給定一個字符串 S，計算 S 的不一樣非空子序列的個數。

由於結果可能很大，因此返回答案模 10^9 + 7.

示例 1：

輸入："abc"
輸出：7
解釋：7 個不一樣的子序列分別是 "a", "b", "c", "ab", "ac", "bc", 以及 "abc"。

示例 2：

輸入："aba"
輸出：6
解釋：6 個不一樣的子序列分別是 "a", "b", "ab", "ba", "aa" 以及 "aba"。

示例 3：

輸入："aaa"
輸出：3
解釋：3 個不一樣的子序列分別是 "a", "aa" 以及 "aaa"。

提示：

1. S 只包含小寫字母。
2. 1 <= S.length <= 2000

---

28ms

 1 class Solution {
 2     func distinctSubseqII(_ S: String) -> Int {
 3         return distinctSubsequence(S.toCharArray(), 1000000007)
 4     }
 5     
 6     func distinctSubsequence(_ a:[Character],_ mod:Int) -> Int
 7     {
 8         var n = a.count
 9         var bucket:[Int] = [Int](repeating: -1,count: 26)
10         
11         var cum:Int = 0
12         for i in 0..<n
13         {
14             var v:Int = cum
15             //'a'的ASCII碼值：97
16             var ind:Int = a[i].ascii - 97
17             if bucket[ind] == -1
18             {
19                 v += 1
20             }
21             else
22             {
23                 v += mod - bucket[ind]
24             }
25             if v >= mod
26             { 
27                 v -= mod
28             }
29             bucket[ind] = cum
30             cum += v
31             if cum >= mod
32             {
33                 cum -= mod
34             }
35         }
36         return cum
37     }
38 }
39 
40 //String擴展方法
41 extension String {
42     func toCharArray() -> [Character]
43     {
44         var arr:[Character] = [Character]()
45         for char in self.characters
46         {
47             arr.append(char)
48         }
49         return arr
50     }
51 }
52 
53 //Character擴展方法  
54 extension Character  
55 {  
56   //轉ASCII整數值(定義小寫爲整數值)
57    var ascii: Int {
58         get {
59             let s = String(self).unicodeScalars
60             return Int(s[s.startIndex].value)
61         }
62     }
63 }