Editorial: First Unique Character

by admin · Oct. 18, 2026

1 Intuition

Count first, then find.

2 Approach

One pass to count frequencies, a second pass to return the first index whose character has count 1.

3 Complexity Analysis

Time O(n), Space O(1) — at most 26 keys.

4 Solution Code

class Solution:
    def firstUniqChar(self, s: str) -> int:
        count = {}
        for c in s:
            count[c] = count.get(c, 0) + 1
        for i, c in enumerate(s):
            if count[c] == 1:
                return i
        return -1

5 Code Walkthrough

The reference solution below implements the approach step by step.