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import java.util.HashMap;
import java.util.Map;
class Solution {
    public String solution(String[] participant, String[] completion) {
        Map<String, Integer> hm = new HashMap<>();
        for(String a : participant) hm.put(a, hm.getOrDefault(a, 0)+1);
        for(String a : completion) hm.put(a, hm.getOrDefault(a, 0)-1);
        for(String a : hm.keySet()){
            if(hm.get(a) != 0) return a;
        }

        return null;

    }
}