LC387 First Unique Character in a String

class Solution:
    def firstUniqChar(self, s: str) -> int:
        # main idea: dictionary.items() return in insertion order 
        # dictionary k:char, v: cnt 
        # find the first k where v == 1
        # time O(N) space O(1) <= O(A) since Character has at most 26 types
        if not s:
            return -1
        d = {}
        for idx,ch in enumerate(s):
            if ch not in d:
                d[ch] = [1,idx]
            else:
                d[ch][0] += 1
        for k,v in d.items():
            if v[0] == 1:
                return v[1]
        return -1