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Fixes: #1709 Longest palindromic subsequence #1713

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Merged
merged 4 commits into from
Oct 14, 2020
Merged

Fixes: #1709 Longest palindromic subsequence #1713

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1bf0fe0
LPS algorithm
matteomessmer Oct 9, 2020
196fcb3
LPS algorithm fix
matteomessmer Oct 9, 2020
9bcb7f5
Fixes: #1709
matteomessmer Oct 9, 2020
60c0291
added tests
matteomessmer Oct 10, 2020
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57 changes: 57 additions & 0 deletions DynamicProgramming/LongestPalindromicSubsequence.java
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Original file line number Diff line number Diff line change
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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;
}
}
33 changes: 33 additions & 0 deletions DynamicProgramming/LongestPalindromicSubsequenceTests.java
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package test;

import static org.junit.jupiter.api.Assertions.*;

import org.junit.jupiter.api.Test;

class LongestPalindromicSubsequenceTests {

@Test
void test1() {
assertEquals(LongestPalindromicSubsequence.LPS(""), "");
}

@Test
void test2() {
assertEquals(LongestPalindromicSubsequence.LPS("A"), "A");
}

@Test
void test3() {
assertEquals(LongestPalindromicSubsequence.LPS("BABCBAB"), "BABCBAB");
}

@Test
void test4() {
assertEquals(LongestPalindromicSubsequence.LPS("BBABCBCAB"), "BABCBAB");
}

@Test
void test5() {
assertEquals(LongestPalindromicSubsequence.LPS("AAAAAAAAAAAAAAAAAAAAAAAA"), "AAAAAAAAAAAAAAAAAAAAAAAA");
}
}