python 打印杨辉三角

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def main():
    num = int(input('Number of rows: '))
    yh = [[]] * num
    for row in range(len(yh)):
        for i in range(num-row-1):
            print(' ', end='')
        yh[row] = [None] * (row+1)

        for col in range(len(yh[row])):
            if col == 0 or col == row:
                yh[row][col] = 1
            else:
                yh[row][col] = yh[row - 1][col] + yh[row - 1][col - 1]
            print(yh[row][col], end=' ')
        print()


if __name__ == '__main__':
    main()