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First Unique Character

Certification: Basic Level Accuracy: 50% Submissions: 2 Points: 5

Write a JavaScript program to find the first non-repeating character in a string and return its index. If no such character exists, return -1. The string contains only lowercase English letters.

Example 1
    
  
  • Input: s = "tutorialspoint"
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  • Output: 1
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  • Explanation: Character 't' at index 0 appears twice in the string. Character 'u' at index 1 appears only once in the string. The first unique character is 'u' at index 1.
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Example 2
    
  
  • Input: s = "codepoint"
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  • Output: 0
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  • Explanation: Character 'C' appears at indices 0 appears only once in the string. The first unique character is 'c' at index 0.
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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)
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  • Space Complexity: O(1) since we only have 26 possible characters
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  • Return -1 if no unique character exists
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StringsWalmartOracle
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Hint
Solution

Solution Hints

    
  
  • Use a hash map or array to count the frequency of each character
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  • Make two passes through the string - first to count, second to find
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  • In the first pass, count the frequency of each character
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  • In the second pass, find the first character with frequency 1
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  • Since only lowercase letters are used, you can use an array of size 26
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  • Map characters to array indices using ASCII values
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  • Return the index of the first character with count 1, or -1 if none exists
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Steps to solve by this approach:

 Step 1: Create an array of size 26 to store frequency count of each lowercase letter.
 Step 2: Initialize all frequency counts to 0.
 Step 3: Make the first pass through the string to count frequency of each character.
 Step 4: Use ASCII value mapping to convert characters to array indices (a=0, b=1, etc.).
 Step 5: Make the second pass through the string to find the first character with frequency 1.
 Step 6: For each character, check its frequency in the count array.
 Step 7: Return the index of the first character with count 1, or -1 if no unique character exists.

Submitted Code :