【hackrank】大小写互换

def swap_case(s):
    def tr(x):
        if x.islower(): 
            return x.upper() 
        elif x.isupper(): 
            return x.lower() 
        else: 
            return x 
    result=''.join([tr(r) for r in list(s)]) 
    return result

参考：https://www.aliyun.com/jiaocheng/465932.html