Java: Find longest Palindromic Substring within a string

Last update on May 21 2025 12:49:19 (UTC/GMT +8 hours)

32. Longest Palindromic Substring

Write a Java program to find the longest Palindromic Substring within a string.

Visual Presentation:

Java String Exercises: Find longest Palindromic Substring within a string

Sample Solution:

Java Code:

// Import the necessary Java utilities.
import java.util.*;

// Define a public class named Main.
public class Main {

    // Define a method to print the substring given start and end indices.
    static void printSubStr(String str1, int l, int h) {
        System.out.println(str1.substring(l, h + 1));
    }

    // Define a method to find the longest palindromic substring.
    static int longPalSubstr(String str1) {
        int n = str1.length();
        boolean table[][] = new boolean[n][n];
        int mLength = 1;

        // Fill the diagonal elements as true.
        for (int i = 0; i < n; ++i)
            table[i][i] = true;

        int strt = 0;

        // Check for palindromes of length 2.
        for (int i = 0; i < n - 1; ++i) {
            if (str1.charAt(i) == str1.charAt(i + 1)) {
                table[i][i + 1] = true;
                strt = i;
                mLength = 2;
            }
        }

        // Check for palindromes of length greater than 2.
        for (int k = 3; k <= n; ++k) {
            for (int i = 0; i < n - k + 1; ++i) {
                int j = i + k - 1;
                if (table[i + 1][j - 1] && str1.charAt(i) == str1.charAt(j)) {
                    table[i][j] = true;

                    if (k > mLength) {
                        strt = i;
                        mLength = k;
                    }
                }
            }
        }

        // Print the longest palindromic substring.
        System.out.print("The longest palindrome substring in the given string is: ");
        printSubStr(str1, strt, strt + mLength - 1);
        return mLength;
    }

    // The main method to execute the code.
    public static void main(String[] args) {
        // Define the input string.
        String str1 = "thequickbrownfoxxofnworbquickthe";

        // Print the given string.
        System.out.println("The given string is: " + str1);

        // Find and print the length of the longest palindromic substring.
        System.out.println("The length of the palindromic substring is: " + longPalSubstr(str1));
    }
}

Sample Output:

The given string is: thequickbrownfoxxofnworbquickthe
The longest palindrome substring in the giv
en string is; brownfoxxofnworb
The length of the palindromic substring is: 16

Flowchart:

For more Practice: Solve these Related Problems:

Write a Java program to find the longest palindromic substring in a string, ignoring case and punctuation.
Write a Java program to locate all palindromic substrings in a string and return the longest one.
Write a Java program to identify the longest palindromic substring in a string using a dynamic programming approach.
Write a Java program to find the longest palindrome in a string by expanding around each character index.

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