Basic Coding - Section 1

1. Reverse a number

public class NumReverse {
    public static int reverse(int number) {
        int reverse = 0;
        int reminder;

        while (number > 0) {
            reminder = number % 10;
            reverse = (reverse * 10) + reminder;
            number = number / 10;
        }
        return reverse;
    }

    public static void main(String[] args) {
        int number = 358;
        System.out.println("Reversed Number is:" + reverse(number));
    }
}

Output

Reversed Number is:853

2. Reverse a number with Recursion

public class NumReverseWithRecursion {
    static int reverse = 0;

    static void reverse(int number){
        if(number<=0)
            return;
        int reminder = number % 10;
        reverse = (reverse * 10) + reminder;
        reverse(number / 10);
    }

    public static void main(String[] args) {
        int number = 1234;
        reverse(number);
        System.out.print("Reversed Number is: " + reverse);
    }
}

Output:

Reversed Number is: 4321

3. Reverse using StringBuilder

public class StringBuilderReverse {
    public static void main(String[] args) {
        int number = 123456;
        String temp = ""+number;

        StringBuilder sb = new StringBuilder(temp);
        StringBuilder str = sb.reverse();
        System.out.println("Reversed String is: "+str.toString());
    }
}

Output:

Reversed String is: 654321

Basic Coding - Section 2

1. Reverse a String

 public static String reverseString(String s) {
        return new StringBuilder(s).reverse().toString();
 }

2. FizzBuzz (1–100)

public static void fizzBuzz() {
        for (int i = 1; i <= 100; i++) {
            if (i % 15 == 0) System.out.println("FizzBuzz");
            else if (i % 3 == 0) System.out.println("Fizz");
            else if (i % 5 == 0) System.out.println("Buzz");
            else System.out.println(i);
        }
    }

3. Two Sum

public static int[] twoSum(int[] nums, int target) {
        Map<Integer, Integer> map = new HashMap<>();
        for (int i = 0; i < nums.length; i++) {
            int complement = target - nums[i];
            if (map.containsKey(complement)) {
                return new int[]{map.get(complement), i};
            }
            map.put(nums[i], i);
        }
        return new int[0];
    }

4. Palindrome Check

public static boolean isPalindrome(String s) {
        int i = 0, j = s.length() - 1;
        while (i < j) {
            if (s.charAt(i) != s.charAt(j)) return false;
            i++; j--;
        }
        return true;
    }

5. Missing Number

public static int missingNumber(int[] nums) {
        int n = nums.length;
        int expectedSum = n * (n + 1) / 2;
        int actualSum = 0;
        for (int num : nums) actualSum += num;
        return expectedSum - actualSum;
    }

6. Detect Duplicates

public static boolean hasDuplicates(int[] nums) {
    Set<Integer> set = new HashSet<>();
    for (int num : nums) {
        if (!set.add(num)) return true;
    }
    return false;
}

7. Reverse Linked List

    static class ListNode {
        int val;
        ListNode next;
        ListNode(int val) { this.val = val; }
    }
    public static ListNode reverseList(ListNode head) {
        ListNode prev = null;
        while (head != null) {
            ListNode next = head.next;
            head.next = prev;
            prev = head;
            head = next;
        }
        return prev;
    }

8. Binary Search

    public static int binarySearch(int[] nums, int target) {
        int left = 0, right = nums.length - 1;
        while (left <= right) {
            int mid = left + (right - left) / 2;
            if (nums[mid] == target) return mid;
            if (nums[mid] < target) left = mid + 1;
            else right = mid - 1;
        }
        return -1;
    }

9. Merge Sort

    public static void mergeSort(int[] arr, int l, int r) {
        if (l < r) {
            int m = (l + r) / 2;
            mergeSort(arr, l, m);
            mergeSort(arr, m + 1, r);
            merge(arr, l, m, r);
        }
    }
    private static void merge(int[] arr, int l, int m, int r) {
        int n1 = m - l + 1, n2 = r - m;
        int[] L = new int[n1], R = new int[n2];
        for (int i = 0; i < n1; i++) L[i] = arr[l + i];
        for (int j = 0; j < n2; j++) R[j] = arr[m + 1 + j];
        int i = 0, j = 0, k = l;
        while (i < n1 && j < n2) {
            arr[k++] = (L[i] <= R[j]) ? L[i++] : R[j++];
        }
        while (i < n1) arr[k++] = L[i++];
        while (j < n2) arr[k++] = R[j++];
    }

10. Second Largest Number

    public static int secondLargest(int[] nums) {
        int max = Integer.MIN_VALUE, second = Integer.MIN_VALUE;
        for (int num : nums) {
            if (num > max) {
                second = max;
                max = num;
            } else if (num > second && num != max) {
                second = num;
            }
        }
        return second;
    }

11. Check Prime Number

    public static boolean isPrime(int n) {
        if (n <= 1) return false;
        for (int i = 2; i * i <= n; i++) {
            if (n % i == 0) return false;
        }
        return true;
    }

12. Fibonacci (Recursion)

    public static int fibonacci(int n) {
        if (n <= 1) return n;
        return fibonacci(n - 1) + fibonacci(n - 2);
    }

13. Count Words in a Sentence

    public static int countWords(String s) {
        s = s.trim();
        if (s.isEmpty()) return 0;
        return s.split("\\s+").length;
    }

14. Shuffle an Array

    public static void shuffleArray(int[] a) {
        Random rand = new Random();
        for (int i = 0; i < a.length; i++) {
            int j = rand.nextInt(a.length);
            int temp = a[i];
            a[i] = a[j];
            a[j] = temp;
        }
    }

15. Reverse String Without Built-In

    public static String reverseManual(String s) {
        char[] c = s.toCharArray();
        for (int i = 0; i < c.length / 2; i++) {
            char temp = c[i];
            c[i] = c[c.length - 1 - i];
            c[c.length - 1 - i] = temp;
        }
        return new String(c);
    }

16. Remove Spaces

    public static String removeSpaces(String s) {
        return s.replaceAll("\\s", "");
    }

17. Trim Leading and Trailing Spaces

   public static String trimSpaces(String s) {
       return s.strip();
   }

18. Sort an Array

    public static void sortArray(int[] a) {
        Arrays.sort(a);
    }

19. Check If Only Odds

    public static boolean onlyOdds(List<Integer> list) {
        return list.stream().allMatch(x -> x % 2 != 0);
    }

20. Factorial

    public static long factorial(int n) {
        return (n <= 1) ? 1 : n * factorial(n - 1);
    }

21. Swap Two Numbers Without Temp

    public static void swap(int a, int b) {
        a = a + b;
        b = a - b;
        a = a - b;
        System.out.println("a=" + a + " b=" + b);
    }

22. Count Distinct Characters

    public static Map<Character, Integer> countDistinctChars(String s) {
        Map<Character, Integer> map = new HashMap<>();
        for (char c : s.toCharArray()) {
            map.put(c, map.getOrDefault(c, 0) + 1);
        }
        return map;
    }

23. Immutable String Demo

    public static void immutableDemo() {
        String s1 = "Java";
        String s2 = s1;
        s1 = "Python";
        System.out.println("s1: " + s1); // Python
        System.out.println("s2: " + s2); // Java
    }

24. Character Pyramid

    public static void printPyramid(int n) {
        for (int i = 1; i <= n; i++) {
            for (int j = 0; j < n - i; j++) System.out.print(" ");
            for (int j = 0; j < 2 * i - 1; j++) System.out.print("*");
            System.out.println();
        }
    }

25. Exception Handling

    public static void fileReadDemo() {
        try {
            FileInputStream fis = new FileInputStream("file.txt");
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }
    }

26. Deadlock Example

    static class Deadlock {
        final Object lock1 = new Object();
        final Object lock2 = new Object();

        public void method1() {
            synchronized (lock1) {
                synchronized (lock2) {
                    System.out.println("Method 1");
                }
            }
        }

        public void method2() {
            synchronized (lock2) {
                synchronized (lock1) {
                    System.out.println("Method 2");
                }
            }
        }
    }

27. Enum Example

    enum State { START, RUNNING, WAITING, DEAD }

28. Record Example (Java 16+)

    record Person(String name, int age) {}

29. Text Blocks Example (Java 15+)

    public static void textBlockExample() {
        String text = """
                Line1
                Line2
                """;
        System.out.println(text);
    }

30. Max Depth of Binary Tree

    static class TreeNode {
        int val;
        TreeNode left, right;
        TreeNode(int val) { this.val = val; }
    }
    public static int maxDepth(TreeNode root) {
        return (root == null) ? 0 : 1 + Math.max(maxDepth(root.left), maxDepth(root.right));
    }