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159. Longest Substring with At Most Two Distinct Characters 👍

  • Time: $O(n)$
  • Space: $O(128) = O(1)$
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class Solution {
 public:
  int lengthOfLongestSubstringTwoDistinct(string s) {
    int ans = 0;
    int distinct = 0;
    vector<int> count(128);

    for (int l = 0, r = 0; r < s.length(); ++r) {
      if (++count[s[r]] == 1)
        ++distinct;
      while (distinct == 3)
        if (--count[s[l++]] == 0)
          --distinct;
      ans = max(ans, r - l + 1);
    }

    return ans;
  }
};

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class Solution {
  public int lengthOfLongestSubstringTwoDistinct(String s) {
    int ans = 0;
    int distinct = 0;
    int[] count = new int[128];

    for (int l = 0, r = 0; r < s.length(); ++r) {
      if (++count[s.charAt(r)] == 1)
        ++distinct;
      while (distinct == 3)
        if (--count[s.charAt(l++)] == 0)
          --distinct;
      ans = Math.max(ans, r - l + 1);
    }

    return ans;
  }
}

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class Solution:
  def lengthOfLongestSubstringTwoDistinct(self, s: str) -> int:
    ans = 0
    distinct = 0
    count = [0] * 128

    l = 0
    for r, c in enumerate(s):
      count[ord(c)] += 1
      if count[ord(c)] == 1:
        distinct += 1
      while distinct == 3:
        count[ord(s[l])] -= 1
        if count[ord(s[l])] == 0:
          distinct -= 1
        l += 1
      ans = max(ans, r - l + 1)

    return ans