Given a string S, the task is to find the string which is lexicographically smallest and not a subsequence of the given string S.
Examples:
Input: S = “abcdefghijklmnopqrstuvwxyz”
Output:
aa
Explanation:
String “aa” is the lexicographically smallest string which is not present in the given string as a subsequence.Input: S = “aaaa”
Output:
aaab
Explanation:
String “aaa” is the lexicographically smallest string which is not present in the given string as a subsequence.
Approach: The idea is to solve the given problem is to find the frequency of the character ‘a’ in the given string (say f). If the value of f is less than the length of the string, then the answer would be the string formed by concatenating ‘a’ f+1 number of times as aaa….(f+1 times) will be the required lexicographically smallest string which is not a subsequence. Otherwise, the answer would be the string formed by concatenating ‘a’ (f-1 times) and ‘b’ at the end of the string.
// 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 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 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# 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 |
<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> |
aa
Time Complexity: O(N)
Auxiliary Space: O(1)