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Lexicographically smallest string possible by performing K operations on a given string
  • Difficulty Level : Medium
  • Last Updated : 15 Feb, 2021
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Given a string S of size N and a positive integer K, the task is to perform atmost K operations on string S to make it lexicographically smallest possible. In one operation, swap S[i] and S[j] and then change S[i] to any character, given 1 ≤ i < j ≤ N.

Examples:

Input: S = “geek”, K = 5
Output: aaaa
Explanation: 
In 1st operation: take i = 1 and j = 4, swap S[1] and S[4] and then change S[1] to ‘a’. Modified string = “aeeg”.
In 2nd operation: take i = 2 and j=4, swap S[2] and S[4] and then change S[2] to ‘a’. Modified string = “aaee”.
In 3rd operation: take i = 3 and j = 4, swap S[3] and S[4] and then change S[3] to ‘a’. Modified string = “aaae”.
In 4th operation: take i = 3 and j = 4, swap S[3] and S[4] and then change S[3] to ‘a’. Modified string = “aaaa”.

Input: S = “geeksforgeeks”, K = 6
Output: aaaaaaeegeeks
Explanation: After 6 operations, lexicographically smallest string will be “aaaaaaeegeeks”.

Approach: For K≥N, the lexicographically smallest possible string will be ‘a’ repeated N times, since, in N operations, all the characters of S can be changed to ‘a’. For all other cases, the idea is to iterate the string S using variable i, find a suitable j for which S[j]>S[i], and then convert S[j] to S[i] and S[i] to ‘a’. Continue this process while K>0.



Follow the steps below to solve the problem:

  • If K ≥ N, convert every character of string S to ‘a’ and print the string, S.
  • Otherwise:

Below is the implementation of the above approach:

C++




// C++ program to implement the above approach
#include <bits/stdc++.h>
using namespace std;
 
// Function to find the lexicographically
// smallest possible string by performing
// K operations on string S
void smallestlexicographicstring(string s, int k)
{
 
    // Store the size of string, s
    int n = s.size();
 
    // Check if k>=n, if true, convert
    // every character to 'a'
    if (k >= n) {
        for (int i = 0; i < n; i++) {
            s[i] = 'a';
        }
        cout << s;
        return;
    }
 
    // Iterate in range[0, n - 1] using i
    for (int i = 0; i < n; i++) {
 
        // When k reaches 0, break the loop
        if (k == 0) {
            break;
        }
 
        // If current character is 'a',
        // continue
        if (s[i] == 'a')
            continue;
 
        // Otherwise, iterate in the
        // range [i + 1, n - 1] using j
        for (int j = i + 1; j < n; j++) {
 
            // Check if s[j] > s[i]
            if (s[j] > s[i]) {
 
                // If true, set s[j] = s[i]
                // and break out of the loop
                s[j] = s[i];
                break;
            }
 
            // Check if j reaches the last index
            else if (j == n - 1)
                s[j] = s[i];
        }
 
        // Update S[i]
        s[i] = 'a';
 
        // Decrement k by 1
        k--;
    }
 
    // Print string
    cout << s;
}
 
// Driver Code
int main()
{
 
    // Given String, s
    string s = "geeksforgeeks";
 
    // Given k
    int k = 6;
 
    // Function Call
    smallestlexicographicstring(s, k);
 
    return 0;
}

Java




// Java program to implement the above approach
public class GFG
{
   
    // Function to find the lexicographically
    // smallest possible string by performing
    // K operations on string S
    static void smallestlexicographicstring(char[] s, int k)
    {
        
        // Store the size of string, s
        int n = s.length;
        
        // Check if k>=n, if true, convert
        // every character to 'a'
        if (k >= n)
        {
            for (int i = 0; i < n; i++)
            {
                s[i] = 'a';
            }
            System.out.print(s);
            return;
        }
        
        // Iterate in range[0, n - 1] using i
        for (int i = 0; i < n; i++)
        {
        
            // When k reaches 0, break the loop
            if (k == 0)
            {
                break;
            }
        
            // If current character is 'a',
            // continue
            if (s[i] == 'a')
                continue;
        
            // Otherwise, iterate in the
            // range [i + 1, n - 1] using j
            for (int j = i + 1; j < n; j++)
            {
        
                // Check if s[j] > s[i]
                if (s[j] > s[i])
                {
        
                    // If true, set s[j] = s[i]
                    // and break out of the loop
                    s[j] = s[i];
                    break;
                }
        
                // Check if j reaches the last index
                else if (j == n - 1)
                    s[j] = s[i];
            }
        
            // Update S[i]
            s[i] = 'a';
        
            // Decrement k by 1
            k--;
        }
        
        // Print string
        System.out.print(s);
    }
     
  // Driver code
    public static void main(String[] args)
    {
       
        // Given String, s
        char[] s = ("geeksforgeeks").toCharArray();
        
        // Given k
        int k = 6;
        
        // Function Call
        smallestlexicographicstring(s, k);
    }
}
 
// This code is contributed by divyesh072019.

Python3




# Python3 program to implement the above approach
 
# Function to find the lexicographically
# smallest possible string by performing
# K operations on string S
def smallestlexicographicstring(s, k):
 
    # Store the size of string, s
    n = len(s)
 
    # Check if k>=n, if true, convert
    # every character to 'a'
    if (k >= n):
        for i in range(n):
         
            s[i] = 'a';
         
        print(s, end = '')
        return;
     
 
    # Iterate in range[0, n - 1] using i
    for i in range(n):
 
        # When k reaches 0, break the loop
        if (k == 0):
            break;
         
        # If current character is 'a',
        # continue
        if (s[i] == 'a'):
            continue;
 
        # Otherwise, iterate in the
        # range [i + 1, n - 1] using j
        for j in range(i + 1, n):
 
            # Check if s[j] > s[i]
            if (s[j] > s[i]):
 
                # If true, set s[j] = s[i]
                # and break out of the loop
                s[j] = s[i];
                break;
         
 
            # Check if j reaches the last index
            elif (j == n - 1):
                s[j] = s[i];
     
 
        # Update S[i]
        s[i] = 'a';
 
        # Decrement k by 1
        k -= 1
 
    # Print string
    print(''.join(s), end = '');
 
 
# Driver Code
if __name__=='__main__':
 
    # Given String, s
    s = list("geeksforgeeks");
 
    # Given k
    k = 6;
 
    # Function Call
    smallestlexicographicstring(s, k);
     
    # This code is contributed by rutvik_56.

C#




// C# program to implement the above approach
using System;
using System.Collections.Generic;
class GFG
{
     
    // Function to find the lexicographically
    // smallest possible string by performing
    // K operations on string S
    static void smallestlexicographicstring(char[] s, int k)
    {
       
        // Store the size of string, s
        int n = s.Length;
       
        // Check if k>=n, if true, convert
        // every character to 'a'
        if (k >= n)
        {
            for (int i = 0; i < n; i++)
            {
                s[i] = 'a';
            }
            Console.Write(s);
            return;
        }
       
        // Iterate in range[0, n - 1] using i
        for (int i = 0; i < n; i++)
        {
       
            // When k reaches 0, break the loop
            if (k == 0)
            {
                break;
            }
       
            // If current character is 'a',
            // continue
            if (s[i] == 'a')
                continue;
       
            // Otherwise, iterate in the
            // range [i + 1, n - 1] using j
            for (int j = i + 1; j < n; j++)
            {
       
                // Check if s[j] > s[i]
                if (s[j] > s[i])
                {
       
                    // If true, set s[j] = s[i]
                    // and break out of the loop
                    s[j] = s[i];
                    break;
                }
       
                // Check if j reaches the last index
                else if (j == n - 1)
                    s[j] = s[i];
            }
       
            // Update S[i]
            s[i] = 'a';
       
            // Decrement k by 1
            k--;
        }
       
        // Print string
        Console.Write(s);
    }
 
  // Driver code
  static void Main()
  {
     
    // Given String, s
    char[] s = ("geeksforgeeks").ToCharArray();
   
    // Given k
    int k = 6;
   
    // Function Call
    smallestlexicographicstring(s, k);
  }
}
 
// This code is contributed by divyeshrabadiya07.

 
 

Output: 
aaaaaaeegeeks

 

Time complexity: O(N2)
Auxiliary space: O(1) 

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