压缩字符串
输入字符串str
例如 HHHEELLLOOOO压缩为H3E2L3O4
例子：
None ->None
'' -> ''
'ABBC' -> AB2C

假设：
字符串仅有ascii组成
大小写敏感

寻找连续出现的子串，并计算出现次数

def Compress(self, string):
    alist = []
    count = 0
    if string is None:
        return None
    if string == '':
        return ''
    alist.append(string[0])
    for i in range(1, len(string)):
        if string[i] == alist[-1]:
            count = count + 1
        else:
            if count != 0:
                alist.append(str(count + 1))
            alist.append(string[i])
            count = 0
    if count != 0:
        alist.append(str(count+1))
    return ''.join(alist)