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++
// 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 Sstring 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 codeint main(){ // Input string S = "abcdefghijklmnopqrstuvwxyz"; int N = S.length(); // Function call cout << smallestNonSubsequence(S, N) << endl; return 0;} |
Java
// Java program for the above approachimport java.util.*;public class GFG{// Function to find lexicographically smallest string that// is not a subsequence of Sstatic 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 Codepublic 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 Sdef 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 codeif __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 programusing 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 Sfunction 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// Inputlet S = "abcdefghijklmnopqrstuvwxyz";let N = S.length;// Function calldocument.write(smallestNonSubsequence(S, N),"</br>");// This code is contributed by shinjanpatra</script> |
Output
aa
Time Complexity: O(N)
Auxiliary Space: O(1)
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