387. First Unique Character in a String

class Solution:
    def firstUniqChar(self, s):
        """
        :type s: str
        :rtype: int
        """
        a_dict = {}
        b_dict = {}
        for i in range(len(s)):
            if s[i] not in a_dict:
                a_dict[s[i]] = i
            else:
                a_dict[s[i]] = -1
        for (a_key, a_value) in a_dict.items():
            if a_dict[a_key] != -1:
                b_dict[a_key] = a_value
        if b_dict == {}:
            return -1
        else:
            return min(b_dict.values())