offer35--第一个只出现一次的字符

# coding=utf-8def firstnum(s):    if s==None or len(s)==0:        return -1    alphapet={}#定义一个空字典    alist=list(s)#将字符串放到列表里    for item in alist:        if item not in alphapet.keys():            alphapet[item]=0#开始都不在字典里，所以初始个数设为0        alphapet[item]+=1    for item in alist:        if alphapet[item]==1:            return item    return itemif __name__ == '__main__':    print firstnum('aabccdf')