如果有重复，就-1，没重复，就存index，找的时候找不为-1且最小的index

# -*- coding:utf-8 -*-
class Solution:
    # 返回对应char
    def __init__(self):
        self.index = 0
        self.dic = {}
    def FirstAppearingOnce(self):
        # write code here
        index = self.index
        res = "#"
        for k, v in self.dic.items():
            if v != -1 and v < index:
                res = k
                index = v
        return res
    def Insert(self, char):
        # write code here
        if char in self.dic:
            self.dic[char] = -1
        else:
            self.dic[char] = self.index
        self.index += 1