Leetcode 387. First Unique Character in a String

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] * 26
        sStart = [0] * 26
        cnts = 0
        for c in s:
            sCnts[ord(c) - ord('a')] += 1
            if sCnts[ord(c) - ord('a')] == 1:
                sStart[ord(c) - ord('a')] = cnts
            cnts += 1
        
        index = -1
        for i in range(26):
            if sCnts[i] == 1 and (index < 0 or index > sStart[i]):
                index = sStart[i]
        
        return index