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Intermediate Java Questions

1. Reverse a String​

public String reverse(String input) {
    return new StringBuilder(input).reverse().toString();
}

2. Check if a number is prime​

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

3. Fibonacci series (using recursion)​

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

4. Factorial using recursion​

public int factorial(int n) {
    if (n <= 1) return 1;
    return n * factorial(n - 1);
}

5. Check palindrome string​

public boolean isPalindrome(String str) {
    return str.equals(new StringBuilder(str).reverse().toString());
}

6. Find duplicate characters in a string​

public Set<Character> findDuplicates(String str) {
    Set<Character> chars = new HashSet<>();
    Set<Character> duplicates = new HashSet<>();
    for (char c : str.toCharArray()) {
        if (!chars.add(c)) duplicates.add(c);
    }
    return duplicates;
}

7. Find missing number in an array​

public int findMissing(int[] nums, int n) {
    int total = n * (n + 1) / 2;
    for (int num : nums) total -= num;
    return total;
}

8. Swap two numbers without temp​

public void swap(int a, int b) {
    a = a + b;
    b = a - b;
    a = a - b;
}

9. Check Armstrong number​

public boolean isArmstrong(int n) {
    int sum = 0, temp = n;
    while (n > 0) {
        int digit = n % 10;
        sum += digit * digit * digit;
        n /= 10;
    }
    return temp == sum;
}

10. Count vowels in a string​

public int countVowels(String str) {
    return (int) str.toLowerCase().chars()
        .filter(c -> "aeiou".indexOf(c) != -1)
        .count();
}

11. Check Anagram​

public boolean isAnagram(String s1, String s2) {
    char[] a = s1.toCharArray(), b = s2.toCharArray();
    Arrays.sort(a); Arrays.sort(b);
    return Arrays.equals(a, b);
}

12. Find 2nd largest number in array​

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

13. Remove duplicates from array​

public int[] removeDuplicates(int[] arr) {
    return Arrays.stream(arr).distinct().toArray();
}

14. Find all pairs with given sum​

public List<int[]> findPairs(int[] arr, int sum) {
    Set<Integer> set = new HashSet<>();
    List<int[]> result = new ArrayList<>();
    for (int num : arr) {
        if (set.contains(sum - num)) {
            result.add(new int[]{num, sum - num});
        }
        set.add(num);
    }
    return result;
}

15. Reverse a LinkedList​

public ListNode reverse(ListNode head) {
    ListNode prev = null, curr = head;
    while (curr != null) {
        ListNode next = curr.next;
        curr.next = prev;
        prev = curr;
        curr = next;
    }
    return prev;
}

16. Detect loop in LinkedList (Floyd’s Cycle)​

public boolean hasCycle(ListNode head) {
    ListNode slow = head, fast = head;
    while (fast != null && fast.next != null) {
        slow = slow.next;
        fast = fast.next.next;
        if (slow == fast) return true;
    }
    return false;
}

17. Merge two sorted arrays​

public int[] mergeSorted(int[] a, int[] b) {
    int[] res = new int[a.length + b.length];
    int i=0, j=0, k=0;
    while (i < a.length && j < b.length) {
        res[k++] = (a[i] < b[j]) ? a[i++] : b[j++];
    }
    while (i < a.length) res[k++] = a[i++];
    while (j < b.length) res[k++] = b[j++];
    return res;
}

18. Find GCD of two numbers​

public int gcd(int a, int b) {
    return b == 0 ? a : gcd(b, a % b);
}

19. Check if string is numeric​

public boolean isNumeric(String str) {
    return str.matches("-?\\d+(\\.\\d+)?");
}

20. Find frequency of characters​

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

21. Java 8: Filter even numbers​

List<Integer> evens = list.stream()
    .filter(n -> n % 2 == 0)
    .collect(Collectors.toList());

22. Sort Map by values​

map.entrySet().stream()
   .sorted(Map.Entry.comparingByValue())
   .forEach(System.out::println);

23. Sum of elements in array using streams​

int sum = Arrays.stream(arr).sum();

24. Java Singleton class​

public class Singleton {
    private static final Singleton instance = new Singleton();
    private Singleton() {}
    public static Singleton getInstance() {
        return instance;
    }
}

25. Interface default method​

interface Demo {
    default void show() {
        System.out.println("Default method");
    }
}

26. Convert List to Map​

Map<Integer, String> map = list.stream()
    .collect(Collectors.toMap(e -> e.id, e -> e.name));

27. Optional Usage​

Optional<String> opt = Optional.ofNullable(value);
opt.ifPresent(System.out::println);

28. Check leap year​

public boolean isLeap(int year) {
    return (year % 4 == 0 && year % 100 != 0) || (year % 400 == 0);
}

29. Convert int to binary​

public String toBinary(int n) {
    return Integer.toBinaryString(n);
}

30. Count words in a string​

public int wordCount(String str) {
    return str.trim().split("\\s+").length;
}