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2351. First Letter to Appear Twice 👍

  • Time: $O(n)$
  • Space: $O(26) = O(1)$
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class Solution {
 public:
  char repeatedCharacter(string s) {
    vector<bool> seen(26);

    for (const char c : s) {
      if (seen[c - 'a'])
        return c;
      seen[c - 'a'] = true;
    }

    throw;
  }
};

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class Solution {
  public char repeatedCharacter(String s) {
    boolean[] seen = new boolean[26];

    for (final char c : s.toCharArray()) {
      if (seen[c - 'a'])
        return c;
      seen[c - 'a'] = true;
    }

    throw new IllegalArgumentException();
  }
}

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class Solution:
  def repeatedCharacter(self, s: str) -> str:
    seen = [False] * 26

    for c in s:
      if seen[ord(c) - ord('a')]:
        return c
      seen[ord(c) - ord('a')] = True