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First Unique Character in a String

Difficulty: Easy

Problem Description

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

Key Insights

  • The problem requires identifying the first character in a string that appears only once.
  • A common approach to solve this is to use a hash table (or dictionary) to count occurrences of each character.
  • After counting, a second pass through the string is needed to determine the index of the first unique character.

Space and Time Complexity

Time Complexity: O(n), where n is the length of the string. We traverse the string twice: once for counting and once for finding the unique character. Space Complexity: O(1), since the maximum number of different characters is limited (only lowercase English letters).

Solution

The solution uses a hash table to count occurrences of each character in the string. After the counting is done, we iterate through the string again to find the first character that has a count of one. The index of this character is returned. If no such character exists, -1 is returned.

Code Solutions

def firstUniqChar(s: str) -> int:
    # Create a dictionary to count the occurrences of each character
    count = {}
    
    # Count each character in the string
    for char in s:
        count[char] = count.get(char, 0) + 1
    
    # Find the first unique character
    for index, char in enumerate(s):
        if count[char] == 1:
            return index  # Return the index of the first unique character
    
    return -1  # If no unique character is found
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