LengthOfLongestSubstringTwoDistinct.java

class LengthOfLongestSubstringTwoDistinct {
    public int lengthOfLongestSubstringTwoDistinct(String str) {
        /*
        Runtime: 50 ms, faster than 81.07% of Java online submissions for Longest Substring with At Most Two Distinct Characters.
Memory Usage: 43.4 MB, less than 85.49% of Java online submissions for Longest Substring with At Most Two Distinct Characters.
        */
        if (str == null || str.length() == 0)
      throw new IllegalArgumentException();

    int windowStart = 0, maxLength = 0;
    Map<Character, Integer> charFrequencyMap = new HashMap<>();
    // in the following loop we'll try to extend the range [windowStart, windowEnd]
    for (int windowEnd = 0; windowEnd < str.length(); windowEnd++) {
      char rightChar = str.charAt(windowEnd);
      charFrequencyMap.put(rightChar, charFrequencyMap.getOrDefault(rightChar, 0) + 1);
      // shrink the sliding window, until we are left with 'k' distinct characters in the frequency map
      while (charFrequencyMap.size() > 2) {
        char leftChar = str.charAt(windowStart);
        charFrequencyMap.put(leftChar, charFrequencyMap.get(leftChar) - 1);
        if (charFrequencyMap.get(leftChar) == 0) {
          charFrequencyMap.remove(leftChar);
        }
        windowStart++; // shrink the window
      }
      maxLength = Math.max(maxLength, windowEnd - windowStart + 1); // remember the maximum length so far
    }

    return maxLength;
    }
}