[LeetCode] 1100. Find K-Length Substrings With No Repeated Characters

Given a string S, return the number of substrings of length K with 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

长度为 K 的无重复字符子串。又是滑动窗口类型的题。但是这个题问的是长度只能是exact K个字符，既不是至多也不是至少。可以用76题的模板做但是需要注意一些细节。

时间O(n)

空间O(n) - hashset

Java实现

class Solution {
    public int numKLenSubstrNoRepeats(String S, int K) {
        // corner case
        int len = S.length();
        if (len < K) {
            return 0;
        }

        // normal case
        int start = 0;
        int end = 0;
        int res = 0;
        HashSet<Character> set = new HashSet<>();
        while (end < len) {
            while (set.contains(S.charAt(end))) {
                set.remove(S.charAt(start));
                start++;
            }
            set.add(S.charAt(end));
            end++;
            if (end - start == K) {
                res++;
                set.remove(S.charAt(start));
                start++;
            }
        }
        return res;
    }
}

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