'''
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: str) -> int:
        visited = set()
        for i in range(len(s)):
            if s[i] not in visited:
                visited.add(s[i])
                if s.count(s[i]) == 1:
                    return i
        return -1
