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有字符串"64,63,62,61"中每个数字加4，应用chr函数，再将其反转成"A-B-C-D"
要知道chr(61+4) == 'A'，刚好能完成转换
方法一:
>>> s = '64,63,62,61'
>>> a = s.split(',')
>>> a.reverse()
>>> a
['61', '62', '63', '64']
>>> b = [chr(int(x)+4) for x in a]
>>> b
['A', 'B', 'C', 'D']
>>> '-'.join(b)
'A-B-C-D'方法二:
>>> s = '64,63,62,61'
>>> '-'.join(map(lambda x:chr(int(x)+4), s.split(',')[::-1]))
'A-B-C-D'方法三:
>>> s = '64,63,62,61'
>>> '-'.join(map(chr, [int(i)+4 for i in s.split(',')[::-1]]))
'A-B-C-D'