First Unique Character in a String

Given a string, find the first non-repeating character in it and return it's index. If it doesn't exist, return -1.

Examples:

s = "leetcode"
return 0.

s = "loveleetcode",
return 2.
Note: You may assume the string contain only lowercase letters.

class Solution:
    def firstUniqChar(self, s):
        """
        :type s: str
        :rtype: int
        """
        d = {}
        cur = 0
        for i in s:  # 用一个字典记录所有只出现一次的字母的index，多次出现的字母index置-1
            if i not in d:
                d[i] = cur
            else:
                d[i] = -1
            cur += 1
        _min = len(s)
        for k, v in d.items():  # 找到只出现一次的字母的最小的index
            if v != -1 and v < _min:
                _min = v
        return _min if _min != len(s) else -1  # 如果没找到（_min没变化），则返回-1