First Unique Character in a String

Tested Python solution for LeetCode 387 with 20 pytest cases. Generate a practice environment with lcpy.

LeetCode 387, Easy. Topics: Hash Table, String, Queue, Counting. View on LeetCode.

Generate this problem as a practice environment: tested reference solution, 20 parametrized pytest cases, and a playground notebook:

lcpy gen -n 387   # by problem number
lcpy gen -s first_unique_character_in_a_string   # by problem name

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Problem

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

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Examples

Input: s = "leetcode"
Output: 0
Explanation: The character 'l' at index 0 is the first character that does not occur at any other index.

Input: s = "loveleetcode"
Output: 2

Input: s = "aabb"
Output: -1

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Constraints

• 1 <= s.length <= 10^5
• s consists of only lowercase English letters.

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Solution

Reference implementation from solution.py on GitHub, full suite in test_solution.py:

from collections import Counter


class Solution:
    # Time: O(n)
    # Space: O(1)
    def first_uniq_char(self, s: str) -> int:
        counts = Counter(s)
        for i, ch in enumerate(s):
            if counts[ch] == 1:
                return i
        return -1

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Complexity

Time	Space
O(n)	O(1)

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Tags

NeetCode All.