# Make largest palindrome by changing at most K-digits

• Difficulty Level : Medium
• Last Updated : 16 Jun, 2022

Given a string containing all digits, we need to convert this string to a palindrome by changing at most K digits. If many solutions are possible then print lexicographically largest one.
Examples:

Input   : str = “43435”
k = 3
Output  : "93939"
Explanation:
Lexicographically largest palindrome
after 3 changes is "93939"

Input :  str = “43435”
k = 1
Output : “53435”
Explanation:
Lexicographically largest palindrome
after 3 changes is “53435”

Input  : str = “12345”
k = 1
Output : "Not Possible"
Explanation:
It is not possible to make str palindrome
after 1 change.

Approach:

1. Solve this problem using two pointers method. We start from left and right and if both digits are not equal then we replace the smaller value with larger value and decrease k by 1. S
2. top when the left and right pointers cross each other, after they stop if value of k is negative, then it is not possible to make string palindrome using k changes. If k is positive, then we can further maximize the string by looping once again in the same manner from left and right and converting both the digits to 9 and decreasing k by 2.
3. If k value remains to 1 and string length is odd then we make the middle character as 9 to maximize whole value.
Below is the implementation of above approach:

## C++

 // C++ program to get largest palindrome changing // atmost K digits #include using namespace std;   // Returns maximum possible // palindrome using k changes string maximumPalinUsingKChanges(string str, int k) {     string palin = str;       // Initialize l and r by leftmost and     // rightmost ends     int l = 0;     int r = str.length() - 1;       //  first try to make string palindrome     while (l < r) {                 // Replace left and right character by         // maximum of both         if (str[l] != str[r]) {             palin[l] = palin[r] =                   max(str[l], str[r]);             k--;         }         l++;         r--;     }       // If k is negative then we can't make     // string palindrome     if (k < 0)         return "Not possible";       l = 0;     r = str.length() - 1;       while (l <= r) {                 // At mid character, if K>0 then change         // it to 9         if (l == r) {             if (k > 0)                 palin[l] = '9';         }           // If character at lth (same as rth) is         // less than 9         if (palin[l] < '9') {             /* If none of them is changed in the                previous loop then subtract 2 from K                and convert both to 9 */             if (k >= 2 && palin[l] == str[l]                 && palin[r] == str[r]) {                 k -= 2;                 palin[l] = palin[r] = '9';             }               /*  If one of them is changed                 in the previous                 loop then subtract 1 from K                 (1 more is                 subtracted already) and make them 9  */             else if (k >= 1                      && (palin[l] != str[l]                          || palin[r] != str[r])) {                 k--;                 palin[l] = palin[r] = '9';             }         }         l++;         r--;     }       return palin; }   //  Driver code to test above methods int main() {     string str = "43435";     int k = 3;     cout << maximumPalinUsingKChanges(str, k);     return 0; }

## Java

 // Java program to get largest palindrome changing // atmost K digits   import java.text.ParseException;   class GFG {     // Returns maximum possible   // palindrome using k changes   static String maximumPalinUsingKChanges(String str,                                           int k)   {     char palin[] = str.toCharArray();     String ans = "";       // Initialize l and r by leftmost and     // rightmost ends     int l = 0;     int r = str.length() - 1;       // First try to make String palindrome     while (l < r) {         // Replace left and right character by       // maximum of both       if (str.charAt(l) != str.charAt(r)) {         palin[l] = palin[r] = (char)Math.max(           str.charAt(l), str.charAt(r));         k--;       }       l++;       r--;     }       // If k is negative then we can't make     // String palindrome     if (k < 0) {       return "Not possible";     }       l = 0;     r = str.length() - 1;       while (l <= r) {         // At mid character, if K>0 then change       // it to 9       if (l == r) {         if (k > 0) {           palin[l] = '9';         }       }         // If character at lth (same as rth) is       // less than 9       if (palin[l] < '9') {           /* If none of them is changed in the             previous loop then subtract 2 from K             and convert both to 9 */         if (k >= 2 && palin[l] == str.charAt(l)             && palin[r] == str.charAt(r)) {           k -= 2;           palin[l] = palin[r] = '9';         }           /* If one of them is changed in the         previous loop then subtract         1 from K (1 more         is subtracted already) and make them 9 */         else if (k >= 1                  && (palin[l] != str.charAt(l)                      || palin[r]                      != str.charAt(r))) {           k--;           palin[l] = palin[r] = '9';         }       }       l++;       r--;     }     for (int i = 0; i < palin.length; i++)       ans += palin[i];     return ans;   }     // Driver code to test above methods   public static void main(String[] args)     throws ParseException   {     String str = "43435";     int k = 3;     System.out.println(       maximumPalinUsingKChanges(str, k));   } } // This code is contributed by 29ajaykumar

## C#

 // C# program to get largest palindrome changing // atmost K digits   using System; public class GFG {     // Returns maximum possible   // palindrome using k changes   static String maximumPalinUsingKChanges(String str,                                           int k)   {     char[] palin = str.ToCharArray();     String ans = "";           // Initialize l and r by leftmost and     // rightmost ends     int l = 0;     int r = str.Length - 1;       // First try to make String palindrome     while (l < r) {               // Replace left and right character by       // maximum of both       if (str[l] != str[r]) {         palin[l] = palin[r]           = (char)Math.Max(str[l], str[r]);         k--;       }       l++;       r--;     }       // If k is negative then we can't make     // String palindrome     if (k < 0) {       return "Not possible";     }       l = 0;     r = str.Length - 1;       while (l <= r) {               // At mid character, if K>0 then change       // it to 9       if (l == r) {         if (k > 0) {           palin[l] = '9';         }       }         // If character at lth (same as rth) is       // less than 9       if (palin[l] < '9') {                   /* If none of them is changed in the         previous loop then subtract 2 from K         and convert both to 9 */         if (k >= 2 && palin[l] == str[l]             && palin[r] == str[r]) {           k -= 2;           palin[l] = palin[r] = '9';         }                   /* If one of them is changed in the         previous loop then subtract 1 from K (1 more         is subtracted already) and make them 9 */         else if (k >= 1                  && (palin[l] != str[l]                      || palin[r] != str[r])) {           k--;           palin[l] = palin[r] = '9';         }       }       l++;       r--;     }     for (int i = 0; i < palin.Length; i++)       ans += palin[i];     return ans;   }     // Driver code to test above methods   public static void Main()   {     String str = "43435";     int k = 3;     Console.Write(maximumPalinUsingKChanges(str, k));   } } // This code is contributed by Rajput-Ji

## Python

 # Python3 program to get largest palindrome changing # atmost K digits   # Returns maximum possible # palindrome using k changes def maximumPalinUsingKChanges(strr, k):     palin = strr[::]       # Initialize l and r by leftmost and     # rightmost ends     l = 0     r = len(strr) - 1       # first try to make palindrome     while (l <= r):           # Replace left and right character by         # maximum of both         if (strr[l] != strr[r]):             palin[l] = palin[r] =                    max(strr[l], strr[r])               # print(strr[l],strr[r])             k -= 1         l += 1         r -= 1       # If k is negative then we can't make     # palindrome     if (k < 0):         return "Not possible"       l = 0     r = len(strr) - 1       while (l <= r):           # At mid character, if K>0 then change         # it to 9         if (l == r):             if (k > 0):                 palin[l] = '9'           # If character at lth (same as rth) is         # less than 9         if (palin[l] < '9'):               # If none of them is changed in the             # previous loop then subtract 2 from K             # and convert both to 9             if (k >= 2 and palin[l] == strr[l] and                            palin[r] == strr[r]):                 k -= 2                 palin[l] = palin[r] = '9'               # If one of them is changed in the previous             # loop then subtract 1 from K (1 more is             # subtracted already) and make them 9             elif (k >= 1 and (palin[l] != strr[l] or                               palin[r] != strr[r])):                 k -= 1                 palin[l] = palin[r] = '9'           l += 1         r -= 1       return palin     # Driver code st = "43435" strr = [i for i in st] k = 3 a = maximumPalinUsingKChanges(strr, k) print("".join(a))   # This code is contributed by mohit kumar 29

## Javascript



Output:

93939

Time complexity: O(n)

Auxiliary Space: O(1)

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