[해시] 완주하지 못한 선수 (JAVA)

import java.util.*;

class Solution {
    public String solution(String[] participant, String[] completion) {
        String answer = "";
        
        Map<String, Integer> participantMap = new HashMap<>();
        for(String p : participant) {
            participantMap.put(p, participantMap.getOrDefault(p, 0)+1);
        }
        
        for(String c : completion) {
            if(participantMap.containsKey(c)) {
                Integer count = participantMap.get(c);
                participantMap.put(c, count-1);
            }
        }
        
        for(String key : participantMap.keySet()) {
            if(participantMap.get(key) != 0) {
                answer += key;
            }
        }
        return answer;
    }
}