Check if a String is Palindrome (Java)

Check if a String is Palindrome (Java)

In this post, we will learn how to check whether a given string is a palindrome using Java. We will solve the problem using two different techniques and handle multiple input strings.

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What is a Palindrome?

A string is called a palindrome if it reads the same forward and backward.

Examples:

ABCBA  → Palindrome
MADAM  → Palindrome
HELLO  → Not a Palindrome

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Approaches Used

• Naive Approach: Reverse the string and compare
• Efficient Approach: Two Pointer Technique

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Java Code Implementation

public class StringPalindrome {

    public static void main(String[] args) {

        String[] inputs = {"ABCBA", "MADAM", "HELLO", "LEVEL"};

        for (String str : inputs) {
            System.out.println("String: " + str);
            System.out.println("Naive Check: " + isPalindromeNaive(str));
            System.out.println("Efficient Check: " + isPalindromeEfficient(str));
            System.out.println();
        }
    }

    // Naive approach using reverse
    private static boolean isPalindromeNaive(String str) {
        StringBuilder reverse = new StringBuilder(str);
        reverse.reverse();
        return str.equals(reverse.toString());
    }

    // Efficient approach using two pointers
    private static boolean isPalindromeEfficient(String str) {

        int left = 0;
        int right = str.length() - 1;

        while (left < right) {
            if (str.charAt(left) != str.charAt(right)) {
                return false;
            }
            left++;
            right--;
        }
        return true;
    }
}

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Explanation

1. Naive Approach (Reverse String)

• Reverse the given string
• Compare it with the original string
• If both are equal, the string is a palindrome

Technique Used: String Reversal

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2. Efficient Approach (Two Pointer Technique)

• Initialize two pointers: one at the start and one at the end
• Compare characters at both pointers
• Move pointers inward until they cross
• If all characters match, it is a palindrome

Technique Used: Two Pointer Technique

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Sample Output

String: ABCBA
Naive Check: true
Efficient Check: true

String: MADAM
Naive Check: true
Efficient Check: true

String: HELLO
Naive Check: false
Efficient Check: false

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Time & Space Complexity

Approach	Time Complexity	Space Complexity
Naive	O(n)	O(n)
Efficient	O(n)	O(1)

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Interview Tip

In interviews, the Two Pointer Technique is preferred because it uses constant extra space and is more efficient.

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Category: Strings
Difficulty: Easy
Technique: Two Pointers