[LeetCode] First Letter to Appear Twice

풀이

class Solution {
    public char repeatedCharacter(String s) {
        Set<Character> seen = new HashSet<>();
        for(int i=0;i<s.length();i++) {
            if(seen.contains(s.charAt(i)))
                return s.charAt(i);
            seen.add(s.charAt(i));
        }
        return ' ';
    }
}