Practice/Goldman Sachs/Leetcode 387. First Unique Character in a String

Leetcode 387. First Unique Character in a String

CodingOptional

Problem

Given a string, find the index of the first character that appears exactly once in the string. If no such character exists, return -1.

Your task is to identify the leftmost character in the string that has no duplicates anywhere else in the string.

Requirements

1. Return the index (0-based) of the first non-repeating character
2. If all characters repeat or the string is empty, return -1
3. The string consists only of lowercase English letters
4. The solution should be efficient for reasonably large inputs

Constraints

• 1 ≤ length of string ≤ 100,000
• The string contains only lowercase English letters (a-z)
• Time complexity should be O(n) where n is the length of the string
• Space complexity should be O(1) since we only have 26 possible characters

Examples

Example 1:

Input: s = "programming" Output: 0 Explanation: The character 'p' appears only once and is at index 0

Example 2:

Input: s = "aabbccdd" Output: -1 Explanation: Every character appears exactly twice, so there is no unique character

Example 3:

Input: s = "xyzzyx" Output: -1 Explanation: All characters appear more than once

Example 4:

Input: s = "statistics" Output: 3 Explanation: The character 't' at index 3 is the first character that appears only once