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Add Longest Common Subsequence algorithm (DP) #188

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Oct 13, 2017
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Add Longest Common Subsequence algorithm (DP) #188

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66 changes: 66 additions & 0 deletions Dynamic Programming/LongestCommonSubsequence.java
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Original file line number Diff line number Diff line change
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class LongestCommonSubsequence {

public static String getLCS(String str1, String str2) {

//At least one string is null
if(str1 == null || str2 == null)
return null;

//At least one string is empty
if(str1.length() == 0 || str2.length() == 0)
return "";

String[] arr1 = str1.split("");
String[] arr2 = str2.split("");

//lcsMatrix[i][j] = LCS of first i elements of arr1 and first j characters of arr2
int[][] lcsMatrix = new int[arr1.length + 1][arr2.length + 1];

for(int i = 0; i < arr1.length + 1; i++)
lcsMatrix[i][0] = 0;
for(int j = 1; j < arr2.length + 1; j++)
lcsMatrix[0][j] = 0;
for(int i = 1; i < arr1.length + 1; i++) {
for(int j = 1; j < arr2.length + 1; j++) {
if(arr1[i-1].equals(arr2[j-1])) {
lcsMatrix[i][j] = lcsMatrix[i-1][j-1] + 1;
} else {
lcsMatrix[i][j] = lcsMatrix[i-1][j] > lcsMatrix[i][j-1] ? lcsMatrix[i-1][j] : lcsMatrix[i][j-1];
}
}
}
return lcsString(str1, str2, lcsMatrix);
}

public static String lcsString (String str1, String str2, int[][] lcsMatrix) {
StringBuilder lcs = new StringBuilder();
int i = str1.length(),
j = str2.length();
while(i > 0 && j > 0) {
if(str1.charAt(i-1) == str2.charAt(j-1)) {
lcs.append(str1.charAt(i-1));
i--;
j--;
} else if(lcsMatrix[i-1][j] > lcsMatrix[i][j-1]) {
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@Santoshjonnakuti Santoshjonnakuti May 31, 2021 •
edited
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if lcsMatrix[i-1][j] is equal to lcsMatrix[i][j-1] then there will be more than 1 Longest Common Subsequence.
But in the above algorithm only one of the Longest Common Subsequence is printed.

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The general algorithm of LCS tries to compute the length of the longest subsequence and print any of them.
Printing all the longest common subsequences is not common, that's why I kept it this way.

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Okay Thanks @rahul1995 bro!

i--;
} else {
j--;
}
}
return lcs.reverse().toString();
}

public static void main(String[] args) {
String str1 = "DSGSHSRGSRHTRD";
String str2 = "DATRGAGTSHS";
String lcs = getLCS(str1, str2);

//Print LCS
if(lcs != null) {
System.out.println("String 1: " + str1);
System.out.println("String 2: " + str2);
System.out.println("LCS: " + lcs);
System.out.println("LCS length: " + lcs.length());
}
}
}