[Hash]完走できなかった選手

public static String solution(String[] participant, String[] completion) {

    String answer = "";
    HashMap<String, Integer> hm = new HashMap<>();
    for (String player : participant) hm.put(player, hm.getOrDefault(player, 0) + 1);
    for (String player : completion) hm.put(player, hm.get(player) - 1);

    for (String key : hm.keySet()) {
        if (hm.get(key) != 0){
            answer = key;
        }
    }
    return answer;
}

Integer getOrDefault(Object key, Integer defaultValue)
key 값이 있으면 해당 value값을 반환하고 없으면 디폴트값을 반환해준다.