Lexicographically smallest string of length N and sum K

Given two integers N and K. The task is to print the lexicographically smallest string of length N consisting of lower-case English alphabates such that the sum of the characters of the string equals to K where ‘a’ = 1, ‘b’ = 2, ‘c’ = 3, ….. and ‘z’ = 26.

Examples:

Input: N = 5, K = 42
Output: aaamz
“aaany”, “babmx”, “aablz” etc. are also valid strings
but “aaamz” is the lexicographically smallest.

Input: N = 3, K = 25
Output: aaw

Approach:

  • Initialize char array of size N and fill all the element by ‘a’.
  • Start traversing from the end of the array and replace the elements of the array by ‘z’ if K ≥ 26 or replace it by the character having ASCII value (K + 97 – 1).
  • At the same time decrease the value of K by replaced element value i.e. for a = 1, b = 2, c = 3, …, z = 26.
  • Also, note that we are subtracting previous element value i.e. (total ‘a’) before current element and adding the same before the end of for loop.
  • Check for K < 0 condition and break the for loop.
  • Return the new string formed by the elements of the char array as the answer.

Below is the implementation of the above approach:

Java

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// Java implementation of the approach
import java.util.Arrays;
  
public class Main {
  
    // Function to return the lexicographically
    // smallest string of length n that
    // satisfies the given condition
    public static char[] lexo_small(int n, int k)
    {
        char arr[] = new char[n];
  
        Arrays.fill(arr, 'a');
  
        // Iteration from the last position
        // in the array
        for (int i = n - 1; i >= 0; i--) {
  
            k -= i;
  
            // If k is a positive integer
            if (k >= 0) {
  
                // 'z' needs to be inserted
                if (k >= 26) {
                    arr[i] = 'z';
                    k -= 26;
                }
  
                // Add the required character
                else {
                    arr[i] = (char)(k + 97 - 1);
                    k -= arr[i] - 'a' + 1;
                }
            }
  
            else
                break;
  
            k += i;
        }
  
        return arr;
    }
  
    // Driver code
    public static void main(String[] args)
    {
        int n = 5, k = 42;
  
        char arr[] = lexo_small(n, k);
  
        System.out.print(new String(arr));
    }
}

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C#

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// C# implementation of the approach 
using System;
  
class GFG
  
    // Function to return the lexicographically 
    // smallest string of length n that 
    // satisfies the given condition 
    public static char[] lexo_small(int n, int k) 
    
        char []arr = new char[n]; 
        int i;
          
        for(i = 0; i < n; i++)
            arr[i] = 'a' ;
  
        // Iteration from the last position 
        // in the array 
        for (i = n - 1; i >= 0; i--) 
        
            k -= i; 
  
            // If k is a positive integer 
            if (k >= 0) 
            
  
                // 'z' needs to be inserted 
                if (k >= 26) 
                
                    arr[i] = 'z'
                    k -= 26; 
                
  
                // Add the required character 
                else
                
                    arr[i] = (char)(k + 97 - 1); 
                    k -= arr[i] - 'a' + 1; 
                
            
  
            else
                break
  
            k += i; 
        
        return arr; 
    
  
    // Driver code 
    public static void Main() 
    
        int n = 5, k = 42; 
  
        char []arr = lexo_small(n, k); 
  
        Console.WriteLine(new string(arr)); 
    
}
  
// This code is contributed by AnkitRai01

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Output:

aaamz


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