문제 링크

나의 CODE

from collections import defaultdict
def solution(participant, completion):
    hash = defaultdict(int)
    for i in participant:
        hash[i] += 1
    for i in completion:
        if i in hash:
            hash[i] -= 1
    for key in hash:
        if hash[key] >= 1:
            return key

Best CODE

import collections


def solution(participant, completion):
    answer = collections.Counter(participant) - collections.Counter(completion)
    return list(answer.keys())[0]

Counter

from collections import Counter
Counter(list) # list의 원소 개수를 딕셔너리 형태로 세준다.
# Counter는 산술 연산이 가능하다.
Counter(['a','a','b']) + Counter(['a','b','b'])
# Result: Counter({'a':3 , 'b':3})