package test;
import java.lang.*; 
import java.io.*; 
import java.util.*; 
  
/**
 * Algorithm explanation https://www.educative.io/edpresso/longest-palindromic-subsequence-algorithm
 */
public class LongestPalindromicSubsequence {
    public static void main(String[] args) {
        String a = "BBABCBCAB";
        String b = "BABCBAB";

        String aLPS = LPS(a);
        String bLPS = LPS(b);

        System.out.println(a + " => " + aLPS);
        System.out.println(b + " => " + bLPS);
    }

    public static String LPS(String original) throws IllegalArgumentException {
        StringBuilder reverse = new StringBuilder(original);
        reverse = reverse.reverse();
        return recursiveLPS(original, reverse.toString());
    }

    private static String recursiveLPS(String original, String reverse) {
        String bestResult = "";
        
        //no more chars, then return empty
        if(original.length() == 0 || reverse.length() == 0) {
            bestResult = "";
        } else {
            
            //if the last chars match, then remove it from both strings and recur 
            if(original.charAt(original.length() - 1) == reverse.charAt(reverse.length() - 1)) {
                String bestSubResult = recursiveLPS(original.substring(0, original.length() - 1), reverse.substring(0, reverse.length() - 1));
                
                bestResult = reverse.charAt(reverse.length() - 1) + bestSubResult;
            } else {
                //otherwise (1) ignore the last character of reverse, and recur on original and updated reverse again
                //(2) ignore the last character of original and recur on the updated original and reverse again
                //then select the best result from these two subproblems.

                String bestSubResult1 = recursiveLPS(original, reverse.substring(0, reverse.length() - 1));
                String bestSubResult2 = recursiveLPS(original.substring(0, original.length() - 1), reverse);
                if(bestSubResult1.length()>bestSubResult2.length()) {
                    bestResult = bestSubResult1;
                } else {
                    bestResult = bestSubResult2;
                }
            }
        }

        return bestResult;
    }
}