Minimum cost to make the String palindrome using ASCII values

Given an input string S, the task is to make it a palindrome and calculate the minimum cost for making it a palindrome, where you are allowed to use any one of the following operations any number of times:

• For replacing the current character with another character the cost will be the absolute difference in ASCII (American Standard Code for Information Interchange) values of these two characters 
• Replace 'a' with both characters and this will cost 10. If the input string is already palindrome then return -1.

Examples:

Input: S = "moon"
Output: 1
Explanation: We need to make 'm' = = 'n' then the input string will become palindrome. So, the cost will be the ASCII value of 'n' - ASCII value of 'm' i.e. 1.

Input: S = "zone"
Output: 11
Explanation: We need to make 'z' == 'e' and the absolute difference between their ASCII values is 21. So we replace both characters with 'a' and it will cost 10 which is more efficient than 21 and for making 'o' == 'n' the cost will be 1. Total cost = 10 + 1 .

Approach: This can be solved with the following idea:

For every pair of characters choose whether their ASCII difference gives an optimized answer or replacing them with 'a' gives the optimized answer.

Below are the steps involved in the implementation of the code:

• Initialize a variable minCost = 0 for tracking the cost.
• Iterate over the input string using a for loop from 0 to n/2(n is the size of the input string) and for every single iteration do the below-mentioned checks.
• If the absolute difference of ASCII values is smaller than 10 then add it to minCost otherwise add 10 to minCost.
• After the for loop is executed then check whether minCost is greater than 0 or not. If yes then return minCost otherwise return -1.

Below is the implementation of the above-mentioned approach in C++:

1

Time Complexity: O(N) 
Auxiliary Space: O(1)