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

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 with the 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: The 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.

Steps to solve this problem:

• Initialize ch to ‘a’.
• If the length of the input string s is less than 26, return false
•  Iterate over the loop 0 to s.size()-1.
  • If ch has reached ‘z’, Break the loop.
  • If the current character in s is less than or equal to ch, replace it with ch and increment ch by one.
•  If ch is less than or equal to ‘z’,  Return false.
•  Return true.

Below is the implementation of the above approach. 

C++

// 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 reference
bool 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 Code
int main()
{
    string str = "aaaaaaaaaaaaaaaaaaaaaaaaaa";
    if (transformString(str))
        cout << str << endl;
    else
        cout << "Not Possible" << endl;
    return 0;
}

Java

// Java Program to transform the given string
// to contain the required subsequence
import 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

# Python3 Program to transform the
# given string to contain the required
# subsequence

# function to transform string with string
# passed as reference
def 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 Code
s = list("aaaaaaaaaaaaaaaaaaaaaaaaaa")
transformString(s)

# This code is contributed by Manish Shaw
# (manishshaw1)

C#

// C# Program to transform the given string
// to contain the required subsequence
using 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
// PHP Program to transform the
// given string to contain the required
// subsequence

// function to transform string with
// string passed as reference
function 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)
?>

Javascript

<script>

// Javascript Plus Plus Program to transform the
// given string to contain the required
// subsequence

// function to transform string with string
// passed as reference
function 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 Code
var 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.
Auxiliary Space: O(1)