https://programmers.co.kr/learn/courses/30/lessons/42576

완주하지 못한 선수.py

def solution(participant, completion):
    answer = ''
    
    count_dict = {}
    for p in participant:
        count_dict[p] = count_dict.get(p, 0) + 1
        
    for c in completion:
        count_dict[c] = count_dict.get(c, 0) - 1
        
        if count_dict[c] == 0:
            del count_dict[c]
    
    answer = list(count_dict.keys())
    return answer[0]