Question

For example :

String is : abcd

shortest palindrome is abcdcba is the solution

longer palindrome can be : abcddcba

another example:

String : aaaab

shortest palindrome is aaaabaaaa

longer palindrome can be aaaaabbaaaa

Restrictions : you can only add characters in the end.

Answer 1

Just append the reverse of initial substrings of the string, from shortest to longest, to the string until you have a palindrome. e.g., for “acbab”, try appending “a” which yields “acbaba”, which is not a palindrome, then try appending “ac” reversed, yielding “acbabca” which is a palindrome.

Update: Note that you don’t have to actually do the append. You know that the substring matches since you just reversed it. So all you have to do is check whether the remainder of the string is a palindrome, and if so append the reverse of the substring. Which is what Ptival wrote symbolically, so he should probably get the credit for the answer. Example: for “acbab”, find the longest suffix that is a palindrome; that is “bab”. Then append the remainder, “ac”, in reverse: ac bab ca.