function solution(participant, completion) {
    let hashMap = new Map();
    
    for (const name of participant) {
        let prevName = hashMap.get(name);
        
        hashMap.set(name, (prevName || 0) + 1);
    }
    // 해시맵을 돌려서 completion에 해당하는 key가 있으면 -1
    for (const arrive of completion) {
        if(hashMap.has(arrive)) {
            hashMap.set(arrive, hashMap.get(arrive) - 1);
        }
    }
    
    for (const [key, value] of hashMap) {
        if (value > 0) {
            return key;
        }
    }
}