练习示例：
1、打印星号正方形
j = 0
while j < 5:
i = 0
while i < 5:
print('*', end='')
i += 1
print()
j += 1
2、打印星号三角形
j = 0
while j < 5:
i = 0
while i <= j:
print('*', end='')
i += 1
print()
j += 1
3、打印九九乘法表
j = 1
while j <= 9:
i = 1
while i <= j:
print(f'{i} * {j} = {i*j}', end='\t')
i += 1
print()
j += 1
