Solution.java

import java.util.*;

class Solution {
    public String solution(String[] participant, String[] completion) {
        String answer = "";
        
        HashMap<String, Integer> hm = new HashMap<>();
        
        for (int i = 0; i < participant.length; i++) {
            if (hm.containsKey(participant[i])) {
                hm.put(participant[i], hm.get(participant[i]) + 1);
            }
            else {
                hm.put(participant[i], 1);
            }
        }
        
        for (int i = 0; i < completion.length; i++) {
            int num = hm.get(completion[i]);
            
            if (num > 1) {
                hm.put(completion[i], num - 1);
            }
            else {
                hm.remove(completion[i]);
            }
        }
        
        for (String keys : hm.keySet()) {
            answer = keys;
        }
        
        return answer;
    }
}

他人の解答

import java.util.HashMap;

class Solution {
    public 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;
                break;
            }
        }
        
        return answer;
    }
}