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package test;
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;
}
}