Merge pull request #1713 from matteomessmer/LongestPalindromicSubsequence

Fixes: #1709 Longest palindromic subsequence
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Ray S 2020-10-14 11:14:26 -04:00 committed by GitHub
commit a19d8b2562
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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;
}
}

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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");
}
}