[Swift]LeetCode1180. 统计只含单一字母的子串｜ Count Substrings with Only One Distinct Letter

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Given a string S, return the number of substrings that have only one distinct letter.

Example 1:

Input: S = "aaaba"
Output: 8
Explanation: The substrings with one distinct letter are "aaa", "aa", "a", "b".
"aaa" occurs 1 time.
"aa" occurs 2 times.
"a" occurs 4 times.
"b" occurs 1 time.
So the answer is 1 + 2 + 4 + 1 = 8.

Example 2:

Input: S = "aaaaaaaaaa"
Output: 55

Constraints:

1 <= S.length <= 1000
S[i] consists of only lowercase English letters.

给你一个字符串 S，返回只含单一字母的子串个数。

示例 1：

输入： "aaaba"
输出： 8
解释： 
只含单一字母的子串分别是 "aaa"， "aa"， "a"， "b"。
"aaa" 出现 1 次。
"aa" 出现 2 次。
"a" 出现 4 次。
"b" 出现 1 次。
所以答案是 1 + 2 + 4 + 1 = 8。

示例 2:

输入： "aaaaaaaaaa"
输出： 55

提示：

1 <= S.length <= 1000
S[i] 仅由小写英文字母组成。

Runtime: 4 ms

Memory Usage: 21 MB

 1 class Solution {
 2     func countLetters(_ S: String) -> Int {
 3         var arr:[Character] = Array(S)
 4         arr.append("#")
 5         var k:Int = 0
 6         var c:Character = "#"
 7         var ans:Int = 0
 8         for i in 0..<arr.count
 9         {
10             if arr[i] == c
11             {
12                 k += 1
13             }
14             else
15             {
16                 if c != "#"
17                 {
18                     ans += k * (k + 1) / 2
19                 }
20                 c = arr[i]
21                 k = 1
22             }
23         }
24         return ans
25     }
26 }