Minimize replacement of characters to its nearest alphabet to make a string palindromic

In this article, we will discuss a fascinating algorithmic problem: "Minimize replacement of characters to its nearest alphabet to make a string palindromic." This problem is intriguing because it involves string manipulation, palindrome checking, and the concept of ASCII values for characters. Let's dive into the problem.

Problem Statement

Given a string of characters, the task is to transform it into a palindrome with the minimum number of replacements. These replacements are made by changing a character to its nearest alphabet.

Understanding the Problem

A palindrome is a word, phrase, number, or other sequences of characters that reads the same backward as forward. Our goal is to minimize the total number of replacements needed to transform a given string into a palindrome.

For example, consider the string "abc". To transform it into a palindrome, we can replace 'c' with 'a' which requires two replacements ('c' to 'b' and 'b' to 'a'). Hence, the minimum number of replacements is 2.

Algorithmic Approach

To solve this problem, we'll use a two-pointer approach. Here are the steps −

  • Initialize two pointers, one at the start and the other at the end of the string.

  • Compare the characters at the pointers.

  • If they are equal, move the pointers inward.

  • If they are not equal, replace the farther character from 'a' with the closer one and increment the count of replacements. Also, move the pointers inward.

  • Repeat steps 2-4 until the start pointer is not less than the end pointer.


Here is the C++ code that implements the above approach −

using namespace std;

int makePalindrome(string str) {
   int len = str.size();
   int res = 0;
   for (int i = 0, j = len - 1; i < j; i++, j--) {
      res += abs(str[i] - str[j]);
   return res;

int main() {
   string str="abcde";
   cout << "Minimum replacements: " << makePalindrome(str);
   return 0;


Minimum replacements: 6

Testcase Example

Let's run an example −

Consider the string "abcde". The above program will output "Minimum replacements: 4". Here's why −

  • Compare 'a' and 'e'. They are not the same, so replace 'e' with 'a'. This needs 4 replacements. Our string is now "abcda".

  • Compare 'b' and 'd'. They are not the same, so replace 'd' with 'b'. This needs 2 replacements. Our string is now "abcba".

  • Now, the string is a palindrome. So, the total minimum replacements are 4 + 2 = 6.

Remember, the number of replacements is calculated as the absolute difference of ASCII values of the characters.


This problem is an excellent example of how simple string manipulation and two-pointer technique can solve relatively complex problems. Understanding such problems not only helps in coding interviews but also improves overall problem-solving skills.

Updated on: 18-May-2023


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