案例：手动输入一个字符串，所有字符反转，大小写相互转

 1 string = input(‘请输入一个字符串：‘)
 2 lists = list(string)
 3 print(‘输入的字符串：‘, lists)
 4 lists.reverse()
 5 print(‘反转后的字符串：‘, lists)
 6 i = 0
 7 while i < len(lists):
 8     if ‘a‘ <= lists[i] <= ‘z‘:
 9         lists[i] = lists[i].upper()
10     elif ‘A‘ <= lists[i] <= ‘Z‘:
11         lists[i] = lists[i].lower()
12     i += 1
13 print(‘大小写改变的字符串：‘, lists)