First Unique Character in a String

Easy · first unique char. Real solution for this problem — same player as the hand-authored deep-dives.

Problem

Read the prompt, flip through the examples, then steal the idea.

LeetCode 387. First Unique Character in a String. The copyrighted prompt is on LeetCode. This page is a working first unique char for this problem: editable input, Python, Go, and a step-by-step animation.

Constraints

—Player inputs are capped so the animation stays readable.
—Match LeetCode's own constraints when you submit there.

Examples

Input

s =: leetcode

Output

Solution

Python and Go. The walkthrough below runs this same code.

Approach 1Optimal▶ Animated

Count, then scan

Count every letter, then return the first index whose count is 1 — or −1.

Two linear passes: the first builds frequencies, the second asks “is this the first singleton?”

Algorithm

Count characters.
Return the first index with count 1.
If none, −1.

Time: O(n)
Space: O(σ)

Solution.py

1class Solution:2    def firstUniqChar(self, s: str) -> int:3        count: dict[str, int] = {}4        for ch in s:5            count[ch] = count.get(ch, 0) + 16        for i, ch in enumerate(s):7            if count[ch] == 1:8                return i9        return -1

Watch this run

Interactive walkthrough

Playing “Count, then scan”. Edit the input, hit Run, then scrub.

Approach: Count, then scan

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