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First Unique Character in a String

Certification: Basic Level Accuracy: 78.95% Submissions: 19 Points: 5

Write a Java program to find the first non-repeating character in a string and return its index. If it doesn't exist, return -1. You must examine each character in the string exactly once.

Example 1
    

  • Input: s = "greenarmy"
    • 

  • Output: 0
    • 

  • Explanation:

      

    • 'g' is the first character and occurs only once
      • 

    

    •
Example 2
    

  • Input: s = "Tutorialspoint"
    • 

  • Output: 2
    • 

  • Explanation:

      

    • 't' appear more than once
      • 

    • 'u' is the first unique character
      • 

    

    •
Constraints
    

  • 1 ≤ s.length ≤ 10^5
    • 

  • s consists of only lowercase English letters
    • 

  • Time Complexity: O(n)
    • 

  • Space Complexity: O(1) or O(k) where k is the character set size
    •
StringsPwCD. E. Shaw
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Hint
Solution

Solution Hints

    
  
  • Use a frequency counter to count the occurrences of each character
    • 
  
  • Iterate through the string once to populate the frequency counter
    • 
  
  • Iterate through the string again to find the first character with a frequency of 1
    • 
  
  • Return the index of that character
    • 
  
  • If no character has a frequency of 1, return -1
    •

Steps to solve by this approach:

 Step 1: Create a frequency array of size 26 to store the count of each lowercase English letter.
 Step 2: Iterate through the string and increment the frequency count for each character.
 Step 3: For each character, calculate its index in the frequency array by subtracting 'a' from it.
 Step 4: After counting the frequency of all characters, iterate through the string again.
 Step 5: For each character, check if its frequency is 1 (meaning it appears only once).
 Step 6: If a character with frequency 1 is found, return its index in the string.
 Step 7: If no character has a frequency of 1, return -1 to indicate that there is no unique character.

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