https://qiita.com/MebukiYamashi/items/bc3efaf7083b103950b0#comment-6f16e8087fd8996aa486

flower.py

def solution(garden):
    days = 0
    # 咲いている花(1)の周囲で咲いていない花(0)、つまり次に咲く花の位置を列挙
    grows = {(ny, nx)
             for y, row in enumerate(garden)
             for x, flower in enumerate(row)
             if flower == 1
             for ny, nx in ((y-1, x), (y+1, x), (y, x-1), (y, x+1))
             if 0 <= ny < len(garden) and 0 <= nx < len(garden[ny])
             if garden[ny][nx] == 0}
    # 次に咲く花がある間
    while grows:
        days += 1
        # 花を咲かせる
        for y, x in grows:
            garden[y][x] = 1
        # 新たに咲いた花の周囲で次に咲く花の位置を列挙する
        grows = {(ny, nx)
                 for y, x in grows
                 for ny, nx in ((y-1, x), (y+1, x), (y, x-1), (y, x+1))
                 if 0 <= ny < len(garden) and 0 <= nx < len(garden[ny])
                 if garden[ny][nx] == 0}
    return days

n = 600
garden = [[0 for i in range(n)] for j in range(n)]
garden[0][0] = 1
garden[5][5] = 1
garden[n-1][n-5] = 1
print(solution(garden))