[Swift]LeetCode940. 不同的子序列 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

---

给定一个字符串 S，计算 S 的不同非空子序列的个数。

因为结果可能很大，所以返回答案模 10^9 + 7.

示例 1：

输入："abc"
输出：7
解释：7 个不同的子序列分别是 "a", "b", "c", "ab", "ac", "bc", 以及 "abc"。

示例 2：

输入："aba"
输出：6
解释：6 个不同的子序列分别是 "a", "b", "ab", "ba", "aa" 以及 "aba"。

示例 3：

输入："aaa"
输出：3
解释：3 个不同的子序列分别是 "a", "aa" 以及 "aaa"。

提示：

1. S 只包含小写字母。
2. 1 <= S.length <= 2000

---

28ms

class Solution {
    func distinctSubseqII(_ S: String) -> Int {
        return distinctSubsequence(S.toCharArray(), 1000000007)
    }
    
    func distinctSubsequence(_ a:[Character],_ mod:Int) -> Int
    {
        var n = a.count
        var bucket:[Int] = [Int](repeating: -1,count: 26)
        
        var cum:Int = 0
        for i in 0..<n
        {
            var v:Int = cum
            //'a'的ASCII码值：97
            var ind:Int = a[i].ascii - 97
            if bucket[ind] == -1
            {
                v += 1
            }
            else
            {
                v += mod - bucket[ind]
            }
            if v >= mod
            { 
                v -= mod
            }
            bucket[ind] = cum
            cum += v
            if cum >= mod
            {
                cum -= mod
            }
        }
        return cum
    }
}

//String扩展方法
extension String {
    func toCharArray() -> [Character]
    {
        var arr:[Character] = [Character]()
        for char in self.characters
        {
            arr.append(char)
        }
        return arr
    }
}

//Character扩展方法  
extension Character  
{  
  //转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)
        }
    }
}