My concise solution in Java

  • 1

    A map to store the right most index of two distinct characters. 1 pointer to store the start index of the substring. The pointer only changes under one condition: the current substring already has two unique characters and the current character is the third character.

    public int lengthOfLongestSubstringTwoDistinct(String s) {
        HashMap<Character, Integer> hm = new HashMap<>();
        int longest = 0;
        int ptr = 0;
        for(int i = 0; i < s.length(); i++) {
            if(!hm.containsKey(s.charAt(i)) && hm.size() == 2) {
                int leftmost = Collections.min(hm.values());
                ptr = s.indexOf((char) hm.keySet().toArray()[0], leftmost);
            hm.put(s.charAt(i), i);
            longest = Math.max(longest, i - ptr + 1);
        return longest;

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