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Find the First Non-Repeating Character in a String

Certification: Intermediate Level Accuracy: 27.27% Submissions: 11 Points: 5

Write a Python program that finds the first non-repeating character in a string.

Example 1
    
    
  • Input: "leetcode"
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  • Output: 'l'
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  • Explanation: 
      
      
    • Step 1: Take the input string "leetcode".
      • 
      
    • Step 2: The first character 'l' only appears once in the string.
      • 
      
    • Step 3: Return 'l' as the result.
      • 
    
    •
Example 2
    
    
  • Input: "loveleetcode"
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  • Output: 'v'
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  • Explanation: 
      
      
    • Step 1: Take the input string "loveleetcode".
      • 
      
    • Step 2: The characters 'l', 'o', 'v', 'e' appear in order, but 'l' and 'e' appear multiple times.
      • 
      
    • Step 3: The first character that appears only once is 'v'.
      • 
      
    • Step 4: Return 'v' as the result.
      • 
    
    •
Constraints
    
    
  • 1 ≤ len(s) ≤ 10^5
    • 
    
  • String contains only lowercase English letters
    • 
    
  • Time Complexity: O(n)
    • 
    
  • Space Complexity: O(1)
    •
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Hint
Solution

Solution Hints

    
  
  • Use a dictionary to count the frequency of each character.
    • 
  
  • Iterate through the string to find the first character with a count of 1.
    • 
  
  • Use the collections.Counter class for efficient counting.
    •

Steps to solve by this approach:

 Step 1: Import Counter from the collections module.
 Step 2: Create a Counter object to count occurrences of each character in the string.
 Step 3: Iterate through each character in the original string (preserving order).
 Step 4: For each character, check if its count is exactly 1.
 Step 5: Return the first character that has a count of 1 (non-repeating).
 Step 6: If no non-repeating character exists, return None.
 Step 7: Example: In "leetcode", 'l' is the first non-repeating character.

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