57 lines
2.2 KiB
Java
57 lines
2.2 KiB
Java
package test;
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import java.lang.*;
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import java.io.*;
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import java.util.*;
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/**
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* Algorithm explanation https://www.educative.io/edpresso/longest-palindromic-subsequence-algorithm
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*/
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public class LongestPalindromicSubsequence {
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public static void main(String[] args) {
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String a = "BBABCBCAB";
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String b = "BABCBAB";
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String aLPS = LPS(a);
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String bLPS = LPS(b);
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System.out.println(a + " => " + aLPS);
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System.out.println(b + " => " + bLPS);
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}
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public static String LPS(String original) throws IllegalArgumentException {
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StringBuilder reverse = new StringBuilder(original);
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reverse = reverse.reverse();
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return recursiveLPS(original, reverse.toString());
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}
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private static String recursiveLPS(String original, String reverse) {
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String bestResult = "";
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//no more chars, then return empty
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if(original.length() == 0 || reverse.length() == 0) {
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bestResult = "";
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} else {
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//if the last chars match, then remove it from both strings and recur
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if(original.charAt(original.length() - 1) == reverse.charAt(reverse.length() - 1)) {
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String bestSubResult = recursiveLPS(original.substring(0, original.length() - 1), reverse.substring(0, reverse.length() - 1));
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bestResult = reverse.charAt(reverse.length() - 1) + bestSubResult;
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} else {
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//otherwise (1) ignore the last character of reverse, and recur on original and updated reverse again
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//(2) ignore the last character of original and recur on the updated original and reverse again
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//then select the best result from these two subproblems.
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String bestSubResult1 = recursiveLPS(original, reverse.substring(0, reverse.length() - 1));
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String bestSubResult2 = recursiveLPS(original.substring(0, original.length() - 1), reverse);
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if(bestSubResult1.length()>bestSubResult2.length()) {
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bestResult = bestSubResult1;
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} else {
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bestResult = bestSubResult2;
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}
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}
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}
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return bestResult;
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}
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} |