159. Longest Substring with At Most Two Distinct Characters

/**
 * Sliding Window.
 *
 * Time Complexity: O(N)
 *
 * Space Complexity: O(1) - Map will max grow to size 3.
 *
 * N = length of the input string.
 */
class Solution {
    public int lengthOfLongestSubstringTwoDistinct(String s) {
        if (s == null) {
            return 0;
        }
        if (s.length() <= 2) {
            return s.length();
        }

        Map<Character, Integer> map = new HashMap();

        int start = 0;
        int end = 0;
        int maxLen = 0;
        int distinctCount = 0;

        while (end < s.length()) {
            char endChar = s.charAt(end);
            map.put(endChar, map.getOrDefault(endChar, 0) + 1);
            if (map.get(endChar) == 1) {
                distinctCount++;
            }
            end++;
            while (distinctCount > 2) {
                char startChar = s.charAt(start);
                map.put(startChar, map.get(startChar) - 1);
                if (map.get(startChar) == 0) {
                    distinctCount--;
                }
                start++;
            }
            maxLen = Math.max(maxLen, end - start);
        }

        return maxLen;
    }
}