[250815] TIL

Today I Learned (2025-08-15)

코딩 테스트 풀기

https://school.programmers.co.kr/learn/courses/30/lessons/12949?language=javascript

우선 행렬 곱셈 어떻게 하는지 까먹었다. 기억을 되살려보자.

[[1,4]
[3,2]
[4,1]]

[[3,3]
[3,3]]

[[]]

3,2 * 2,2 = 3,2 (행렬)
3,3 * 3,3 = 3,3 (행렬)

arr1 행 = [ㅇ][]
arr2 렬 = [][ㅇ]

case 1 ) 3,2 * 2,2 = 3,2 (행렬)

answer[0][0] = arr1[0][0] * arr2[0][0] + arr1[0][1] * arr2[1][0]
answer[0][1] = arr1[0][0] * arr2[0][1] + arr1[0][1] * arr2[1][1]

answer[1][0] = arr1[1][0] * arr2[0][0] + arr1[1][1] * arr2[1][0]
answer[1][1] = arr1[1][0] * arr2[0][1] + arr1[1][1] * arr2[1][1]

answer[2][0] = arr1[2][0] * arr2[0][0] + arr1[2][1] * arr2[1][0]
answer[2][1] = arr1[2][0] * arr2[0][1] + arr1[2][1] * arr2[1][1]

case 2 ) 3,3 * 3,3 = 3,3 (행렬)

answer[0][0] = arr1[0][0] * arr2[0][0] + arr1[0][1] * arr2[1][0] + arr1[0][2] *a arr2[2][0]
answer[0][1] = arr1[0][0] * arr2[0][1] + arr1[0][1] * arr2[1][1] ...
answer[0][2] = arr1[0][0] * arr2[0][2] + arr1[0][1] * arr2[1][2] ...

answer[1][0] = arr1[1][0] * arr2[0][0] + arr1[1][1] * arr2[1][0] ...
answer[1][1] = arr1[1][0] * arr2[0][1] + arr1[1][1] * arr2[1][1] ...
answer[1][2] = arr1[1][0] * arr2[0][2] + arr1[1][1] * arr2[1][2] ...

answer[2][0] = arr1[2][0] * arr2[0][0] + arr1[2][1] * arr2[1][0] ...
answer[2][1] = arr1[2][0] * arr2[0][1] + arr1[2][1] * arr2[1][1] ...
answer[2][2] = arr1[2][0] * arr2[0][2] + arr1[2][1] * arr2[1][2] ...

ㅠㅠ 복잡하다 천천히 생각해보자 ! answer 카운팅은 어떻게 할까?

answer 행은 arr1의 행 길이로 결정된다. answer 열은 arr2 열 길이로 결정된다.

for 행 i add; for 열 j add; for 열 j add += arr1 i j * arr j i answer i j = add

function solution(arr1, arr2) {
  const row = arr1.length;
  const c1 = arr1[0].length;
  const c2 = arr2[0].length;
  let answer = [...new Array(row)].map(() => new Array(c2).fill(0));

  for (let i = 0; i < row; i++) {
    for (let j = 0; j < c2; j++) {
      for (let k = 0; k < c1; k++) {
        answer[i][j] += arr1[i][k] * arr2[k][j];
      }
    }
  }

  return answer;
}

문제 풀이 거의 근접했는데 후반에 뇌가 멈춰서 잘 못했다 .. . ㅠㅠㅠㅠㅠㅠㅠㅠㅠㅠㅠㅠㅠㅠㅠㅠㅠㅠㅠㅠㅠㅠ

아깝다아아