Lexicographically smallest string which is not a subsequence of given string

Last Updated : 01 Feb, 2023

C++

// C++ program for the above approach
#include <bits/stdc++.h>
using namespace std;
// Function to find lexicographically smallest string that
// is not a subsequence of S
string smallestNonSubsequence(string S, int N)
{
    // variable to store frequency of 'a'
    int freq = 0;
    // calculate frequency of 'a'
    for (int i = 0; i < N; i++)
        if (S[i] == 'a')
            freq++;
    // Initialize string consisting of freq number of 'a'
    string ans(freq, 'a');
    // change the last digit to 'b'
    if (freq == N)
        ans[freq - 1] = 'b';
    // add another 'a' to the string
    else
        ans += 'a';
    // return the answer string
    return ans;
}
// Driver code
int main()
{
    // Input
    string S = "abcdefghijklmnopqrstuvwxyz";
    int N = S.length();
 
    // Function call
    cout << smallestNonSubsequence(S, N) << endl;
    return 0;
}

Java

// Java program for the above approach
import java.util.*;
public class GFG
{
 
// Function to find lexicographically smallest string that
// is not a subsequence of S
static String smallestNonSubsequence(String S, int N)
{
   
    // variable to store frequency of 'a'
    int freq = 0;
   
    // calculate frequency of 'a'
    for (int i = 0; i < N; i++)
        if (S.charAt(i) == 'a')
            freq++;
   
    // Initialize string consisting of freq number of 'a'
 String ans="";
    for(int i = 0; i < f req; i++){
        ans += 'a';
    }
   
    // change the last digit to 'b'
    if (freq == N)
       ans = ans.replace(ans.charAt(freq - 1), 'b');
   
    // add another 'a' to the string
    else
        ans += 'a';
   
    // return the answer string
    return ans;
}
 
// Driver Code
public static void main(String args[])
{
    String S = "abcdefghijklmnopqrstuvwxyz";
    int N = S.length();
 
    // Function call
    System.out.println(smallestNonSubsequence(S, N));
    }
}
 
// This code is contributed by SoumikMondal

Python3

# Python3 program for the above approach
 
# Function to find lexicographically smallest 
# string that is not a subsequence of S
def smallestNonSubsequence(S, N):
     
    # Variable to store frequency of 'a'
    freq = 0
     
    # Calculate frequency of 'a'
    for i in range(N):
        if (S[i] == 'a'):
            freq += 1
             
    # Initialize string consisting
    # of freq number of 'a'
    ans = ""
    for i in range(freq):
        ans += 'a'
 
    # Change the last digit to 'b'
    if (freq == N):
        ans = ans.replace(ans[freq - 1], 'b')
         
    # Add another 'a' to the string
    else:
        ans += 'a'
         
    # Return the answer string
    return ans
 
# Driver code
if __name__ == '__main__':
     
    # Input
    S = "abcdefghijklmnopqrstuvwxyz"
    N = len(S)
 
    # Function call
    print(smallestNonSubsequence(S, N))
     
# This code is contributed by SURENDRA_GANGWAR

C#

// C# program equivalent to the above Java program
using System;
 
class GFG 
{
   
  // Function to find lexicographically smallest string
  // that is not a subsequence of S
  static string SmallestNonSubsequence(string S, int N)
  {
     
    // variable to store frequency of 'a'
    int freq = 0;
 
    // calculate frequency of 'a'
    for (int i = 0; i < N; i++)
      if (S[i] == 'a')
        freq++;
 
    // Initialize string consisting of freq number of
    // 'a'
    string ans = "";
    for (int i = 0; i < freq; i++) {
      ans += 'a';
    }
 
    // change the last digit to 'b'
    if (freq == N)
      ans = ans.Replace(ans[freq - 1], 'b');
 
    // add another 'a' to the string
    else
      ans += 'a';
 
    // return the answer string
    return ans;
  }
 
  // Driver Code
  static void Main(string[] args)
  {
    string S = "abcdefghijklmnopqrstuvwxyz";
    int N = S.Length;
 
    // Function call
    Console.WriteLine(SmallestNonSubsequence(S, N));
  }
}
 
// This code is contributed by phasing17

Javascript

<script>
 
// JavaScript program for the above approach
 
// Function to find lexicographically smallest string that
// is not a subsequence of S
function smallestNonSubsequence(S,N)
{
 
    // variable to store frequency of 'a'
    let freq = 0;
     
    // calculate frequency of 'a'
    for (let i = 0; i < N; i++)
        if (S[i] == 'a')
            freq++;
             
    // Initialize string consisting of freq number of 'a'
    let ans = new Array(freq).fill('a').join('');
    // change the last digit to 'b'
    if (freq == N)
        ans[freq - 1] = 'b';
         
    // add another 'a' to the string
    else
        ans += 'a';
         
    // return the answer string
    return ans;
}
 
// Driver code
 
// Input
let S = "abcdefghijklmnopqrstuvwxyz";
let N = S.length;
 
// Function call
document.write(smallestNonSubsequence(S, N),"</br>");
 
// This code is contributed by shinjanpatra
 
</script>