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First Unique Character in a String

Certification: Basic Level Accuracy: 0% Submissions: 0 Points: 5

Write a C program to find the first non-repeating character in a string and return its index. If it doesn't exist, return -1. The function should return the index of the first character that appears exactly once in the string.

Example 1
    
  
  • Input: s = "Love"
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  • Output: 0
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  • Explanation: 'l' appears only once in the string and it's the first non-repeating character, so we return its index which is 0.
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Example 2
    
  
  • Input: s = "Tutorialspoint"
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  • Output: 1
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  • Explanation: 't' appears twice, 'u' appears once. The first non-repeating character is 'u' at index 1.
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Constraints
    
  
  • 1 ≤ s.length ≤ 10^5
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  • s consists of only lowercase English letters
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  • Time Complexity: O(n) where n is the length of the string
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  • Space Complexity: O(1) since we only need space for a fixed size array of 26 characters
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Hint
Solution

Solution Hints

    
  
  • Use an array to count the frequency of each character in the string
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  • Traverse the string once to fill the frequency array
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  • Traverse the string again to find the first character with a frequency of 1
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  • Return the index of that character
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  • If no such character exists, return -1
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Steps to solve by this approach:

 Step 1: Initialize a frequency array of size 26 to store the count of each lowercase letter.

 Step 2: Traverse the string once and count the frequency of each character.

 Step 3: We subtract 'a' from each character to get its index in the frequency array (e.g., 'a'-'a'=0, 'b'-'a'=1).

 Step 4: Traverse the string again to find the first character with a frequency of 1.

 Step 5: Return the index of the first unique character.

 Step 6: If no unique character is found, return -1.

 Step 7: Time complexity is O(n) because we traverse the string twice, and space complexity is O(1) because the array size is constant.

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