Construct lexicographically smallest palindrome

Given a string of lowercase alphabets. Some of characters of given string got corrupted and are now represented by *. We can replace * with any of lowercase alphabets. You have to construct lexicographically smallest palindrome string. If it is not possible to construct a palindrome print “Not Possible”.

Examples:

Input : str[] = "bc*b" 
Output : bccb

Input : str[] = "bc*a*cb"
Output : bcaaacb

Input : str[] = "bac*cb"
Output : Not Possible

Recommended: Please try your approach on {IDE} first, before moving on to the solution.

Start traversing the string from both end. Say with i=0, j=strlen-1, keep increasing i and decreasing j after every single iteration till i exceeds j. Now at any intermediate position we have five possible case :

str[i] and str[j] both are same and also not equal to ‘*’. In this case simply continue.
str[i] and str[j] both are same and are equal to ‘*’. Here you must fill str[i] = str[j] = ‘a’ for smallest possible palindrome.
str[i] equals to ‘*’ and str[j] is some alphabet. Here fill str[i] = str[j] to make our string a palindrome.
str[j] equals to ‘*’ and str[i] is some alphabet. Here fill str[j] = str[i] to make our string a palindrome.
str[i] is not equals to str[j] and also both are some alphabet. In this case palindrome construction is not possible. So, print “Not Possible” and break from loop.

After i exceeds j means we have got our required palindrome. Else we got “Not possible” as result.

C++

// CPP for constructing smallest palindrome 
#include <bits/stdc++.h> 
using namespace std; 
  
// function for printing palindrome 
string constructPalin(string str, int len) 
{ 
    int i = 0, j = len - 1; 
  
    // iterate till i<j 
    for (; i < j; i++, j--) { 
  
        // continue if str[i]==str[j] 
        if (str[i] == str[j] && str[i] != '*') 
            continue; 
  
        // update str[i]=str[j]='a' if both are '*' 
        else if (str[i] == str[j] && str[i] == '*') { 
            str[i] = 'a'; 
            str[j] = 'a'; 
            continue; 
        } 
  
        // update str[i]=str[j] if only str[i]='*' 
        else if (str[i] == '*') { 
            str[i] = str[j]; 
            continue; 
        } 
  
        // update str[j]=str[i] if only str[j]='*' 
        else if (str[j] == '*') { 
            str[j] = str[i]; 
            continue; 
        } 
  
        // else print not possible and return 
        cout << "Not Possible"; 
        return ""; 
    } 
    return str; 
} 
  
// driver program 
int main() 
{ 
    string str = "bca*xc**b"; 
    int len = str.size(); 
    cout << constructPalin(str, len); 
    return 0; 
} 

Java

// Java for constructing smallest palindrome 
class GFG  
{ 
  
// function for printing palindrome 
static String constructPalin(char []str, int len) 
{ 
    int i = 0, j = len - 1; 
  
    // iterate till i<j 
    for (; i < j; i++, j--)  
    { 
  
        // continue if str[i]==str[j] 
        if (str[i] == str[j] && str[i] != '*') 
            continue; 
  
        // update str[i]=str[j]='a' if both are '*' 
        else if (str[i] == str[j] &&  
                        str[i] == '*')  
        { 
            str[i] = 'a'; 
            str[j] = 'a'; 
            continue; 
        } 
  
        // update str[i]=str[j] if only str[i]='*' 
        else if (str[i] == '*')  
        { 
            str[i] = str[j]; 
            continue; 
        } 
  
        // update str[j]=str[i] if only str[j]='*' 
        else if (str[j] == '*')  
        { 
            str[j] = str[i]; 
            continue; 
        } 
  
        // else print not possible and return 
        System.out.println("Not Possible"); 
        return ""; 
    } 
    return String.valueOf(str); 
} 
  
// Driver code 
public static void main(String[] args)  
{ 
    String str = "bca*xc**b"; 
    int len = str.length(); 
    System.out.println(constructPalin(str.toCharArray(), len)); 
} 
} 
  
// This code is contributed by 29AjayKumar 

PHP

<?php  
// PHP for constructing smallest palindrome 
  
// function for printing palindrome 
function constructPalin($str, $len) 
{ 
    $i = 0; 
    $j = $len - 1; 
  
    // iterate till i<j 
    for (; $i < $j; $i++, $j--) 
    { 
  
        // continue if str[i]==str[j] 
        if ($str[$i] == $str[$j] &&  
            $str[$i] != '*') 
            continue; 
  
        // update str[i]=str[j]='a' if both are '*' 
        else if ($str[$i] == $str[$j] &&  
                 $str[$i] == '*')  
        { 
            $str[$i] = 'a'; 
            $str[$j] = 'a'; 
            continue; 
        } 
  
        // update str[i]=str[j] if only str[i]='*' 
        else if ($str[$i] == '*')  
        { 
            $str[$i] = $str[$j]; 
            continue; 
        } 
  
        // update str[j]=str[i] if only str[j]='*' 
        else if ($str[$j] == '*')  
        { 
            $str[$j] = $str[$i]; 
            continue; 
        } 
  
        // else print not possible and return 
        echo "Not Possible"; 
        return ""; 
    } 
    return $str; 
} 
  
// Driver Code 
$str = "bca*xc**b"; 
$len = strlen($str); 
echo constructPalin($str, $len); 
  
// This code is contributed by ita_c 
?> 

Output:

bcacxcacb

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Recommended: Please try your approach on {IDE} first, before moving on to the solution.

C++

Java

PHP

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