字符串提取字符：不可超过特定范围
训练replace（）替换
split（）分隔字符串，join（）合并
import time
time01 = time.time()
a = ''
for i in range(1000000):
a += 'sxt'
time02 = time.time()
print('耗时：',str(time02-time01))
time03 = time.time()
li = []
for i in range(1000000):
li.append('sxt')
a = ''.join(li)
time04 = time.time()
print('耗时：',str(time04-time03))
字符串添加尽量使用join（），耗时更少
进行逻辑判断与print初始化
