# 2351. First Letter to Appear Twice

• Time: $O(n)$
• Space: $O(26) = O(1)$
  1 2 3 4 5 6 7 8 9 10 11 12 13 14 class Solution { public: char repeatedCharacter(string s) { vector seen(26); for (const char c : s) { if (seen[c - 'a']) return c; seen[c - 'a'] = true; } throw; } }; 
  1 2 3 4 5 6 7 8 9 10 11 12 13 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(); } } 
 1 2 3 4 5 6 7 8 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