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Longest Substring with at Most k Distinct Characters

Certification: Intermediate Level Accuracy: 0% Submissions: 0 Points: 10

Write a C# program to find the length of the longest substring of a string s that contains at most k distinct characters.

Example 1
    
  
  • Input: s = "eceba", k = 2
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  • Output: 3
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  • Explanation: 
    
      
      
    • The substring "ece" contains 2 distinct characters, 'e' and 'c'.
      • 
    
    
  
    •
Example 2
    
  
  • Input: s = "aa", k = 1
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  • Output: 2
    • 
  
  • Explanation: 
    
      
      
    • The substring "aa" contains 1 distinct character, 'a'.
      • 
    
    
  
    •
Constraints
    
  
  • 1 <= s.length <= 5 * 10^4
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  • 0 <= k <= 50
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  • s consists of lowercase English letters
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  • Time Complexity: O(n) where n is the length of the string
    • 
  
  • Space Complexity: O(min(k, m)) where m is the size of the character set
    •
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Hint
Solution

Solution Hints

    
  
  • Use a sliding window approach to track the substring
    • 
  
  • Maintain a dictionary to keep track of the frequency of each character in the current window
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  • When adding a new character would exceed k distinct characters, shrink the window from the left
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  • Keep track of the maximum window size seen so far
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  • The solution should iterate through the string once
    •

Steps to solve by this approach:

 Step 1: Use a sliding window approach with two pointers (left and right).
 Step 2: Maintain a dictionary to keep track of the characters in the current window and their frequencies.
 Step 3: Expand the window by moving the right pointer and adding characters to the dictionary.
 Step 4: When the number of distinct characters exceeds k, shrink the window by moving the left pointer and removing characters from the dictionary as necessary.
 Step 5: Keep track of the maximum window size seen so far.
 Step 6: Return the maximum window size as the result.
 Step 7: Handle edge cases like empty strings or k = 0.

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