Given a stringS, return the number of substrings of lengthKwith no repeated characters.

Example 1:

Input: S = "havefunonleetcode", K = 5
Output: 6
Explanation: 
There are 6 substrings they are : 'havef','avefu','vefun','efuno','etcod','tcode'.

Example 2:

Input: S = "home", K = 5
Output: 0
Explanation: 
Notice K can be larger than the length of S. In this case is not possible to find any substring.
 

Note:

1. 1 <= S.length <= 10^4
2. All characters of S are lowercase English letters.
3. 1 <= K <= 10^4

長度為 K 的無重複字元子串。又是滑動視窗型別的題。但是這個題問的是長度只能是exact K個字元，既不是至多也不是至少。可以用76題的模板做但是需要注意一些細節。

時間O(n)

空間O(n) - hashset

Java實現

 1 class Solution {
 2     public int numKLenSubstrNoRepeats(String S, int K) {
 
 3         // corner case
 4         int len = S.length();
 5         if (len < K) {
 6             return 0;
 7         }
 8 
 9         // normal case
10         int start = 0;
11         int end = 0;
12         int res = 0;
13         HashSet<Character> set = new HashSet<>();
14         while
 
 (end < len) {
15             while (set.contains(S.charAt(end))) {
16                 set.remove(S.charAt(start));
17                 start++;
18             }
19             set.add(S.charAt(end));
20             end++;
21             if (end - start == K) {
22                 res++;
23                 set.remove(S.charAt(start));
24                 start++;
25             }
26         }
27         return res;
28     }
29 }

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