[20250320] BOJ / G5 / 4와 7 / 신희을 #272
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🧷 문제 링크
https://www.acmicpc.net/problem/2877
🧭 풀이 시간
20 분
👀 체감 난이도
✏️ 문제 설명
창영이는 4와 7로 이루어진 수를 좋아한다. 창영이가 좋아하는 수 중에 K번째 작은 수를 구해 출력하는 프로그램을 작성하시오.
🔍 풀이 방법
자릿수의 마지막 번호는 2, (2 + 4), (2 + 4 + 8) 식으로 늘어난다.
N이 포함된 자릿수를 구하고(자릿수를 k라 하면)
이전 자릿수의 마지막 번호(2 + 4) +$2^{k-1}$ 이 N보다 크다면? N의 맨 앞 자리 값이 7이고,
N보다 작다면? N의 맨 앞 자리 값이 4이다.
이를 활용해서 계산한다.
⏳ 회고
엠로 코딩 테스트에서 나온 문제이다.