배울점: 만만하게 봤다가 돌아갔다. 중복을 핸들링하는게 포인트였다ㅏ.

from collections import defaultdict
def solution(participant, completion):
    answer = ''
    
    dic=defaultdict(int)
    for i in completion:
        dic[i] += 1

    for ele in participant:
        dic[ele] -= 1
        if dic[ele] == -1:
            return ele

    return answer