2022.07.24 (lc_6124_the first letter that appears twice)

  Method ： Count

class Solution {
    public char repeatedCharacter(String s) {
        int[] counter = new int[26];
        for (char ch : s.toCharArray()) {
            counter[ch - 'a']++;
            if (counter[ch - 'a'] > 1) {
                return ch;
            }
        }
        return 'a';
    }
}

Method 2 ： Hashtable

class Solution {
    public char repeatedCharacter(String s) {
        Set<Character> set = new HashSet<>();
        for (char ch : s.toCharArray()) {
            if (!set.add(ch)) {
                return ch;
            }
        }
        return 'a';
    }
}