essentially the same as LC340. Longest Substring with at Most K Distinct Characters.

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class Solution {
    public int lengthOfLongestSubstringTwoDistinct(String s) {
        HashMap<Character, Integer> map = new HashMap<>();
        int res = 0, left = 0;
        for (int i = 0; i < s.length(); ++i) {
            char c = s.charAt(i);
            map.put(c, map.getOrDefault(c, 0) + 1);
            
            while (map.size() > 2) {
                char leftChar = s.charAt(left);
                map.put(leftChar, map.get(leftChar) - 1);
                if (map.get(leftChar) == 0) map.remove(leftChar);
                
                left ++;
            }
            res = Math.max(res, i - left + 1);
        }
        
        return res;
    }
}