In the shrinking window part:

        // current window size is from windowStart to windowEnd, overall we have a letter
        // which is repeating 'maxRepeatLetterCount' times, this means we can have a window
        // which has one letter repeating 'maxRepeatLetterCount' times and the remaining
        // letters we should replace. If the remaining letters are more than 'k', it is the
        // time to shrink the window as we are not allowed to replace more than 'k' letters
        if (windowEnd - windowStart + 1 - maxRepeatLetterCount > k) {
            char leftChar = str.charAt(windowStart);
            letterFrequencyMap.put(leftChar, letterFrequencyMap.get(leftChar) - 1);
            windowStart++;
        }

What if the chatAt(windowStart) is a maxRepeatLetter? After removing windowStart character, we still have more than k letter to be replaced, because removing a maxRepeatLetter reduces the macRepeatLetterCount. So in this case, the next step calculating maxLength is not correct, because this substring [windowStart, windowEnd] would require more than k letters to be replaced. Am I missing something? Thanks.