2351. First Letter to Appear Twice

Given a string s consisting of lowercase English letters, return the first letter to appear twice.

Note:

• A letter a appears twice before another letter b if the second occurrence of a is before the second occurrence of b.
• s will contain at least one letter that appears twice.

class Solution {
public:
    char repeatedCharacter(string s) {
        unordered_set<char> us;
        for(auto& ch : s) {
            if(us.count(ch)) return ch;
            us.insert(ch);
        }
        return '#';
    }
};