Pythonの2次元配列を1次元配列に変換する4つの方法

[i for j in a for i in j]

from itertools import chain
list(chain.from_iterable(a))

sum(a, [])

import operator
from functools import reduce
print(reduce(operator.add, a))

def fun(arr, lines):
    total_matrix = [[0] * lines for i in range(lines)]
    """            """
    num = [i for j in arr for i in j]  
    num = sorted(num)
    show_num = 0
    col = lines - 1
    row = lines - 1
    start_line = 0

    def print_ju(start_line, col, row, show_num):
        if row == 0:
            if lines % 2 != 0:
                total_matrix[lines // 2][lines // 2] = lines * lines
        else:
            for i in range(start_line, col):  #      
                total_matrix[start_line][i] = num[show_num]
                show_num += 1
            for i in range(start_line, row, 1):  #      
                total_matrix[i][col] = num[show_num]
                show_num += 1
            for i in range(row, row - col + start_line, -1):  #      
                total_matrix[row][i] = num[show_num]
                show_num += 1
            for i in range(col, col - row + start_line, -1):  #       
                total_matrix[i][row - col + start_line] = num[show_num]
                show_num += 1
            return print_ju(start_line + 1, row - 1, col - 1, show_num)

    print_ju(start_line, row, col, show_num)
    for i in total_matrix:
        for x in i:
            print(format(x, '3'), end=' ')
        print()


if __name__ == '__main__':
    n = int(input('          ：'))
    arr = [[4, 0, 1], [9, 2, 7], [8, 12, 90]]
    fun(arr, n)