# Count of substrings of length K with exactly K distinct characters
###### tags: `Sliding window`, `Amazon OA`

Description:
Given string `str` of the lowercase alphabet and an integer `k`, the task is to count all substrings of length `k` which have exactly `k` distinct characters.

Solution:

public class kLengthDistinctCharSubstring {
    public int countSubstrings(String str, int k) {
        int res = 0;
        int[] arr = new int[26];
        int l = 0;
        int r = 0;

        // change the cnt while moving the pointers
        while (r < str.length()) {
            // count the char at pointer r
            char c = str.charAt(r);
            ++arr[c - 'a'];

            // move left pointer in the following scenarios:
                // 1. number of char at r is larger than 2
                // 2. length of substring > k
            while (arr[c - 'a'] > 1 || r - l + 1 > k) {
                --arr[str.charAt(l++) - 'a'];
            }

            if (r - l + 1 == k) {
                res++;
            }
            r++;
        }
        return res;
    }
}