2022.07.24(LC_6124_第一个出现两次的字母)

 方法：计数

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';
    }
}

方法二：哈希表

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';
    }
}