Java Program for Longest Palindromic Subsequence

For longest Palindromic subsequence, the Java code is as follows −

Example

public class Demo{
   static String longest_seq(String str_1, String str_2){
      int str_1_len = str_1.length();
      int str_2_len = str_2.length();
      char str_1_arr[] = str_1.toCharArray();
      char str_2_arr[] = str_2.toCharArray();
      int L[][] = new int[str_1_len + 1][str_2_len + 1];
      for (int i = 0; i <= str_1_len; i++){
         for (int j = 0; j <= str_2_len; j++){
            if (i == 0 || j == 0){
               L[i][j] = 0;
            }
            else if (str_1_arr[i - 1] == str_2_arr[j - 1]){
               L[i][j] = L[i - 1][j - 1] + 1;
            }
            else{
               L[i][j] = Math.max(L[i - 1][j], L[i][j - 1]);
            }
         }
      }
      int my_index = L[str_1_len][str_2_len];
      char[] longest_seq = new char[my_index + 1];
      int i = str_1_len, j = str_2_len;
      while (i > 0 && j > 0){
         if (str_1_arr[i - 1] == str_2_arr[j - 1]){
            longest_seq[my_index - 1] = str_1_arr[i - 1];
            i--;
            j--;
            my_index--;
         }
         else if (L[i - 1][j] > L[i][j - 1]){
            i--;
         } else {
            j--;
         }
      }
      String my_result = "";
      for (int x = 0; x < longest_seq.length; x++){
         my_result += longest_seq[x];
      }
      return my_result;
   }
   static String longestPalSubseq(String str){
      String rev_str = str;
      rev_str = reverse_str(rev_str);
      return longest_seq(str, rev_str);
   }
   static String reverse_str(String str){
      String my_result = "";
      char[] trial = str.toCharArray();
      for (int i = trial.length - 1; i >= 0; i--){
         my_result += trial[i];
      }
      return my_result;
   }
   public static void main(String[] args){
      String str = "HelloHelloo";
      System.out.println("Longest palindromic subsequence is ");
      System.out.println(longestPalSubseq(str));
   }
}

Output

Longest palindromic subsequence is
llell

A class named Demo contains the function ‘longest_seq’ that declares two strings and two character arrays. The arrays will be iterated over, and the longest palindromic sequence is found, by using dynamic programming technique. In this method, once the value for a specific array is found, it is stored and not re-computed again, thereby making calculations efficient.

A function named ‘longestPalSubseq’ takes the string as parameter, and reverses the string and calls the ‘longest_seq’ function by passing the reversed string. Another function named ‘reverse_str’ is used to reverse the string that is passed as parameter to the function. In the main function, the string is defined, and the function ‘longestPalSubseq’ is called, and output is displayed on the console.