面试题50. 第一个只出现一次的字符

https://leetcode-cn.com/problems/di-yi-ge-zhi-chu-xian-yi-ci-de-zi-fu-lcof/

class Solution(object):
    def firstUniqChar(self, s):
        """
        :type s: str
        :rtype: str
        """
        dic = dict()
        for c in s:
            dic[c] = dic.get(c, 0) + 1
        for k in s:
            if dic[k] == 1:
                return k
        return ' '