Problem

Given a string s, find the first non-repeating character in it and return its index. If it does not exist, return -1.

Algorithm

Use two lists: one list is used to count the letters in “s”; the other list is the position where the letter first appears. Then find the smallest position of the letters appear once.

Code

class Solution:def firstUniqChar(self, s: str) -> int:sCnts = [0] * 26sStart = [0] * 26cnts = 0for c in s:sCnts[ord(c) - ord('a')] += 1if sCnts[ord(c) - ord('a')] == 1:sStart[ord(c) - ord('a')] = cntscnts += 1index = -1for i in range(26):if sCnts[i] == 1 and (index < 0 or index > sStart[i]):index = sStart[i]return index