Transform a string such that it has abcd..z as a subsequence

26 May, 2021
Given a string S of only small English letters. We need to transform the string by making some number of moves (any number of times) to get the string “abcdefghijklmnopqrstuvwxyz” as a subsequence in that string. In one move, you can replace any character of the string to next character in alphabetical order i.e. ‘a’ can be replaced by ‘b’, ‘b’ can be replaced by ‘c’ and so on. Letter ‘z’ cannot be replaced by any character. If it is not possible to get the subsequence out of that string, then print “Not Possible”.Note: Subsequence of a string is the string obtained by deleting some characters at some positions.Examples:  
Input : aaaaaaaaaaaaaaaaaaaaaaaaaa
Output : abcdefghijklmnopqrstuvwxyz
Explanation: Second occurrence of letter 'a' will be 
replaced by 'b', third occurrence of letter 'a' will 
be first replaced by 'b' and then by 'c' and so on.

Input : helloworld
Output : Not Possible
 
This problem can be solved using a greedy approach. The idea is to first observe that in a single move we can increment any character to next character only. That is, ‘c’ can be incremented to ‘d’,’e’,’f’ or any character greater than ‘c’. So we will create a variable ch, initially initialized to ‘a’ and by iterating over the string if we find the current character of the string not greater than ch, we’ll replace it with ch and increase the value of ch in alphabetical order. Since ch is initially equal to ‘a’, we’ll find ‘a’, replace it with ch, increment ch to store ‘b’ and move forward in the string, then if we find any character not greater than ‘b’ replace it with ‘b’ and again increment ch to store ‘c’ and move forward in the string. We will repeat these steps until ch reaches ‘z’ or the entire string is processed. If after traversing the whole string, ch does not reach ‘z’ then it is not possible to obtain the required subsequence.Below is the implementation of the above approach.  
C++
Java
Python3
C#
PHP
Javascript
// C Plus Plus Program to transform the// given string to contain the required// subsequence#include <bits/stdc++.h>using namespace std; // function to transform string with string// passed as referencebool transformString(string& s){    // initializing the variable ch to 'a'    char ch = 'a';     // if the length of string is less than    // 26, we can't obtain the required    // subsequence    if (s.size() < 26)        return false;        for (int i = 0; i < s.size(); i++) {         // if ch has reached 'z', it means we have        // transformed our string such that required        // subsequence can be obtained        if (int(ch) > int('z'))            break;         // current character is not greater than ch,        // then replace it with ch and increment ch        if (s[i] <= ch) {            s[i] = ch;            ch = char(int(ch) + 1);        }    }         if (ch <= 'z')        return false;    return true;} // Driver Codeint main(){    string str = "aaaaaaaaaaaaaaaaaaaaaaaaaa";       if (transformString(str))        cout << str << endl;    else        cout << "Not Possible" << endl;    return 0;}
// Java Program to transform the given string// to contain the required subsequenceimport java.io.*; public class GFG {         // function to transform string with    // string passed as reference    static boolean transformString(StringBuilder s)    {                 // initializing the variable ch to 'a'        char ch = 'a';             // if the length of string is less than        // 26, we can't obtain the required        // subsequence        if (s.length() < 26)            return false;             for (int i = 0; i < s.length(); i++) {                 // if ch has reached 'z', it means            // we have transformed our string            // such that required subsequence            // can be obtained            if ((int)ch > (int)'z')                break;                 // current character is not greater            // than ch, then replace it with            // ch and increment ch            if (s.charAt(i) <= ch) {                s.setCharAt(i, ch);                ch = (char)((int)ch + 1);            }        }                 if (ch <= 'z')            return false;        return true;    }         // Driver Code    public static void main(String args[])    {        StringBuilder str =        new StringBuilder("aaaaaaaaaaaaaaaaaaaaaaaaaa");                  if (transformString(str))            System.out.println(str.toString());        else            System.out.println("Not Possible");    }} // This code is contributed by Manish Shaw// (manishshaw1)
# Python3 Program to transform the# given string to contain the required# subsequence  # function to transform string with string# passed as referencedef transformString(s) :     # initializing the variable ch to 'a'    ch = 'a'       # if the length of string is less than    # 26, we can't obtain the required    # subsequence    if (len(s) < 26) :        return False          for i in range(0, len(s)):           # if ch has reached 'z', it means we have        # transformed our string such that required        # subsequence can be obtained        if (ord(ch) > ord('z')) :            break           # current character is not greater than ch,        # then replace it with ch and increment ch        if (s[i] <= ch) :            s[i] = ch            ch = chr(ord(ch) + 1)           if (ch <= 'z') :        print ("Not Possible")    print ("".join(s))   # Driver Codes = list("aaaaaaaaaaaaaaaaaaaaaaaaaa")transformString(s) # This code is contributed by Manish Shaw# (manishshaw1)
// C# Program to transform the given string// to contain the required subsequenceusing System;using System.Text;using System.Collections.Generic; class GFG {     // function to transform string with string    // passed as reference    static bool transformString(ref StringBuilder s)    {                 // initializing the variable ch to 'a'        char ch = 'a';             // if the length of string is less than        // 26, we can't obtain the required        // subsequence        if (s.Length < 26)            return false;             for (int i = 0; i < s.Length; i++) {                 // if ch has reached 'z', it means we            // have transformed our string such            // that required subsequence can be            // obtained            if ((int)ch > 122)                break;                 // current character is not greater            // than ch, then replace it with ch            // and increment ch            if (s[i] <= ch) {                s[i] = ch;                ch = (char)((int)ch + 1);            }        }                 if (ch <= 'z')            return false;        return true;    }     // Driver Code    public static void Main()    {        StringBuilder str = new        StringBuilder("aaaaaaaaaaaaaaaaaaaaaaaaaa");                 if (transformString(ref str))            Console.WriteLine(str + "\n");        else            Console.WriteLine("Not Possible" + "\n");    }} // This code is contributed by Manish Shaw// (manishshaw1)
<?php// PHP Program to transform the// given string to contain the required// subsequence // function to transform string with// string passed as referencefunction transformString(&$s){         // initializing the variable    // ch to 'a'    $ch = "a";     // if the length of string is less than    // 26, we can't obtain the required    // subsequence    if (strlen($s) < 26)        return false;     for ($i = 0; $i < strlen($s); $i++)    {         // if ch has reached 'z',        // it means we have        // transformed our string        // such that required        // subsequence can be obtained        if (ord($ch) > ord("z"))            break;         // current character is not        // greater than ch, then        // replace it with ch and        // increment ch        if ($s[$i] <= $ch)        {            $s[$i] = $ch;            $ch = chr(ord($ch) + 1);        }    }         if ($ch <= "z")        return false;    return true;} // Driver Code$str = "aaaaaaaaaaaaaaaaaaaaaaaaaa";if (transformString($str))    echo $str;else    echo "Not Possible";     // This code is contributed by// Manish Shaw (manishshaw1)?>
<script> // Javascript Plus Plus Program to transform the// given string to contain the required// subsequence // function to transform string with string// passed as referencefunction transformString(){    // initializing the variable ch to 'a'    var ch = 'a';     // if the length of string is less than    // 26, we can't obtain the required    // subsequence    if (s.length < 26)        return false;        for (var i = 0; i < s.length; i++) {         // if ch has reached 'z', it means we have        // transformed our string such that required        // subsequence can be obtained        if (ch.charCodeAt(0) > 122)            break;         // current character is not greater than ch,        // then replace it with ch and increment ch        if (s[i].charCodeAt(0) <= ch.charCodeAt(0)) {            s[i] = ch;            ch = String.fromCharCode(ch.charCodeAt(0) + 1);        }    }    s = s.join('')    if (ch.charCodeAt(0) <= 'z'.charCodeAt(0))        return false;    return true;} // Driver Codevar s = "aaaaaaaaaaaaaaaaaaaaaaaaaa".split('');   if (transformString(s))    document.write(s);else    document.write( "Not Possible"); </script>
Output:  
abcdefghijklmnopqrstuvwxyz
Time Complexity: O(n), where n is the length of the string. 
manishshaw1
ManasChhabra2
itsok
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Strings
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