Find the Longest Palindromic Substring in Java (Efficient Two-Pointer Approach)

In this tutorial, we’ll learn how to find the longest palindromic substring in a given string using Java. This is a famous interview question asked by companies like Google, Amazon, and Microsoft.

🧠 What is a Palindromic Substring?

A palindrome is a string that reads the same backward as forward. A palindromic substring is a part of the string that is also a palindrome.

📌 Example

Input: "babad"
Output: "bab" or "aba"

Input: "cbbd"
Output: "bb"

✅ Java Program (Expand Around Center Approach)

public class LongestPalindrome {
    public static void main(String[] args) {
        String input = "babad";
        String result = longestPalindrome(input);
        System.out.println("Longest Palindromic Substring: " + result);
    }

    static String longestPalindrome(String s) {
        if (s == null || s.length() < 1) return "";

        int start = 0, end = 0;

        for (int i = 0; i < s.length(); i++) {
            int len1 = expandFromMiddle(s, i, i);       // Odd length
            int len2 = expandFromMiddle(s, i, i + 1);   // Even length
            int len = Math.max(len1, len2);

            if (len > end - start) {
                start = i - (len - 1) / 2;
                end = i + len / 2;
            }
        }

        return s.substring(start, end + 1);
    }

    static int expandFromMiddle(String s, int left, int right) {
        while (left >= 0 && right < s.length() && s.charAt(left) == s.charAt(right)) {
            left--;
            right++;
        }
        return right - left - 1;
    }
}

✅ Output

Longest Palindromic Substring: bab

💡 Interview Tip

This method runs in O(n^2) time and uses O(1) space. You might also be asked to implement it using dynamic programming, which uses extra space but follows a tabulation-based approach.

📚 Conclusion

We explored how to find the longest palindromic substring using the expand-around-center technique in Java. This problem is a favorite in coding interviews and helps sharpen your understanding of string manipulation and two-pointer techniques.