Basic Java Problems¶

This section contains fundamental Java programming problems to help you practice core concepts.

Problem 1: String Reversal¶

Problem Statement¶

Write a Java program to reverse a string without using the built-in reverse() method.

Requirements¶

Input: A string
Output: The reversed string
Constraints: Do not use StringBuffer or StringBuilder reverse() method

Sample Input/Output¶

Input: "Hello, World!"
Output: "!dlroW ,olleH"

Solution Approach¶

Convert the string to a character array
Use two pointers (start and end)
Swap characters at both pointers
Move pointers towards the center
Convert back to string

Solution¶

public class StringReversal {
    public static String reverse(String input) {
        if (input == null || input.isEmpty()) {
            return input;
        }

        char[] chars = input.toCharArray();
        int start = 0;
        int end = chars.length - 1;

        while (start < end) {
            // Swap characters
            char temp = chars[start];
            chars[start] = chars[end];
            chars[end] = temp;

            // Move pointers
            start++;
            end--;
        }

        return new String(chars);
    }
}

Test Cases¶

@Test
public void testReverse() {
    assertEquals("", StringReversal.reverse(""));
    assertEquals("a", StringReversal.reverse("a"));
    assertEquals("ba", StringReversal.reverse("ab"));
    assertEquals("!dlroW ,olleH", StringReversal.reverse("Hello, World!"));
    assertNull(StringReversal.reverse(null));
}

Problem 2: Palindrome Check¶

Problem Statement¶

Write a Java program to check if a given string is a palindrome.

Requirements¶

Input: A string
Output: Boolean indicating if the string is a palindrome
Constraints: Ignore case and non-alphanumeric characters

Sample Input/Output¶

Input: "A man, a plan, a canal: Panama"
Output: true

Input: "race a car"
Output: false

Solution Approach¶

Convert string to lowercase
Remove non-alphanumeric characters
Use two pointers to compare characters from both ends
Return true if all characters match

Solution¶

public class PalindromeCheck {
    public static boolean isPalindrome(String input) {
        if (input == null || input.isEmpty()) {
            return true;
        }

        // Convert to lowercase and remove non-alphanumeric characters
        String cleaned = input.toLowerCase().replaceAll("[^a-z0-9]", "");

        int start = 0;
        int end = cleaned.length() - 1;

        while (start < end) {
            if (cleaned.charAt(start) != cleaned.charAt(end)) {
                return false;
            }
            start++;
            end--;
        }

        return true;
    }
}

Test Cases¶

@Test
public void testIsPalindrome() {
    assertTrue(PalindromeCheck.isPalindrome(""));
    assertTrue(PalindromeCheck.isPalindrome("a"));
    assertTrue(PalindromeCheck.isPalindrome("A man, a plan, a canal: Panama"));
    assertFalse(PalindromeCheck.isPalindrome("race a car"));
    assertTrue(PalindromeCheck.isPalindrome(null));
}

Contributing¶

Feel free to add more problems or improve the existing solutions. See our Contributing Guide for details.