利用python中的字典来记录字符出现次数并找到第一次只出现一次的字符。2*O(n) 
# -*- coding:utf-8 -*-
class Solution:
    def FirstNotRepeatingChar(self, s):
        alpha_cnt={}
        for i in s:
            if i not in alpha_cnt.keys():
                alpha_cnt[i]=1
            elif i in alpha_cnt.keys():
                alpha_cnt[i]+=1
        idx=0
        for j in s:
            if alpha_cnt[j]==1:
                return idx
            idx+=1
        return -1
        # write code here