Q #1) Write a Java Program to reverse a string without using String inbuilt
function.
TWO POINTER ==> START AND END THERE ==> USES WHILE NOT FOR
Q #2) Write a Java Program to swap two numbers without using the third
variable.
ADD-SUB-SUB
CHANGE IN MIDDLE FINGER (B COMES)
Answer:
`public void swapNumbers(int a, int b) {
a = a + b; // A=A^B
b = a - b; // B=A^B
a = a - b; // A=A^B
System.out.println("After swap: a = " + a + ", b = " + b);
}`
Q 3) Write a Java Program to count the number of words in a string using
HashMap
Q #4)** Write a Java Program to iterate HashMap using While and advance for
loop.**
Q #5) Write a Java Program to find whether a number is prime or not in the most efficient way?
Q #6) Write a Java Program to find whether a string or number is
palindrome or not.
Q #7) Write a Java Program for the Fibonacci series in recursion.
Q #8) Write a Java Program to iterate ArrayList using for-loop, while-loop,
and advance for-loop.
import java.util.ArrayList;
public class ArrayListIterationExample {
public static void main(String[] args) {
// Create ArrayList
ArrayList<String> list = new ArrayList<>();
// Add elements
list.add("Java");
list.add("Python");
list.add("C++");
list.add("JavaScript");
// 1️⃣ Using For Loop
System.out.println("Using For Loop:");
for (int i = 0; i < list.size(); i++) {
System.out.println(list.get(i));
}
// 2️⃣ Using While Loop
System.out.println("\nUsing While Loop:");
int i = 0;
while (i < list.size()) {
System.out.println(list.get(i));
i++;
}
// 3️⃣ Using Enhanced For Loop (Advanced For Loop)
System.out.println("\nUsing Enhanced For Loop:");
for (String language : list) {
System.out.println(language);
}
}
}
Q #9) Write a Java Program to find the duplicate characters in a string.
import java.util.HashMap;
public class DuplicateCharacters {
public static void main(String[] args) {
String str = "programming";
HashMap<Character, Integer> map = new HashMap<>();
// Count each character
for (char ch : str.toCharArray()) {
map.put(ch, map.getOrDefault(ch, 0) + 1);
}
// Print duplicates
System.out.println("Duplicate characters:");
for (char ch : map.keySet()) {
if (map.get(ch) > 1) {
System.out.println(ch);
}
}
}
}
Q #10) Write a Java Program to find the second-highest number in an array.
Q #11) Write a Java Program to check Armstrong number.
Q #12) Write a Java Program to remove all white spaces from a string
without using replace().
Q #13) Given an array of integers nums and an integer target, return indices of the two numbers such that they add up to target.
Q #14) Write a program that accepts comma-separated strings, sorts the
strings in ascending order, and outputs the concatenated string of sorted
strings.
Q #15) Given a string s, return true if s is a "good" string, or false otherwise. A string s is good if all characters that appear in s have the same number of occurrences (i.e., the same frequency).
Q #16) Given an array nums and a value val, remove all instances of that
value in-place and return the new length of the array. Do not allocate extra space for another array. You must modify the input array in-place with O(1) extra memory.
Q #17) You are given an integer array nums and an array of queries queries where queries[i] = [val, index]. For each query, add val to nums[index]. Then, return the sum of all even numbers in nums.
public class SumEvenAfterQueries {
public static void main(String[] args) {
int[] nums = {1, 2, 3, 4};
int[][] queries = {{1,0}, {-3,1}, {-4,0}, {2,3}};
int evenSum = 0;
// Step 1: Calculate initial even sum
for (int num : nums) {
if (num % 2 == 0) {
evenSum += num;
}
}
// Step 2: Process queries
for (int[] q : queries) {
int val = q[0];
int index = q[1];
// If old value was even, subtract it
if (nums[index] % 2 == 0) {
evenSum -= nums[index];
}
// Update value
nums[index] += val;
// If new value is even, add it
if (nums[index] % 2 == 0) {
evenSum += nums[index];
}
System.out.println(evenSum);
}
}
}
Q #18) Given two strings s and p, find all the start indices of p's anagrams in s.
import java.util.*;
public class FindAnagrams {
public static void main(String[] args) {
String s = "cbaebabacd";
String p = "abc";
List<Integer> result = new ArrayList<>();
int[] count = new int[26];
// Step 1: Store frequency of p
for (char c : p.toCharArray()) {
count[c - 'a']++;
}
int left = 0, right = 0, needed = p.length();
while (right < s.length()) {
// If current char needed
if (count[s.charAt(right) - 'a'] > 0) {
needed--;
}
count[s.charAt(right) - 'a']--;
right++;
// If window size equals p length
if (needed == 0) {
result.add(left);
}
// Maintain window size
if (right - left == p.length()) {
if (count[s.charAt(left) - 'a'] >= 0) {
needed++;
}
count[s.charAt(left) - 'a']++;
left++;
}
}
System.out.println(result);
}
}
Q #19) Given a string s, find the length of the longest substring without
repeating characters.
import java.util.HashSet;
public class LongestSubstring {
public static void main(String[] args) {
String s = "abcabcbb";
HashSet<Character> set = new HashSet<>();
int left = 0, maxLength = 0;
for (int right = 0; right < s.length(); right++) {
while (set.contains(s.charAt(right))) {
set.remove(s.charAt(left));
left++;
}
set.add(s.charAt(right));
maxLength = Math.max(maxLength, right - left + 1);
}
System.out.println("Longest Length: " + maxLength);
}
}
Q #20) Merge two sorted linked lists and return it as a new sorted list.
class ListNode {
int val;
ListNode next;
ListNode(int val) {
this.val = val;
}
}
public class MergeSortedLists {
public static ListNode merge(ListNode l1, ListNode l2) {
ListNode dummy = new ListNode(0);
ListNode current = dummy;
while (l1 != null && l2 != null) {
if (l1.val < l2.val) {
current.next = l1;
l1 = l1.next;
} else {
current.next = l2;
l2 = l2.next;
}
current = current.next;
}
// Attach remaining nodes
if (l1 != null) current.next = l1;
if (l2 != null) current.next = l2;
return dummy.next;
}
}
Q #21) You are given an n x n 2D matrix representing an image, rotate the image by 90 degrees (clockwise).
`
`
public class RotateImage {
public static void main(String[] args) {
int[][] matrix = {
{1, 2, 3},
{4, 5, 6},
{7, 8, 9}
};
int n = matrix.length;
// Step 1: Transpose (swap rows & columns)
for (int i = 0; i < n; i++) {
for (int j = i; j < n; j++) {
int temp = matrix[i][j];
matrix[i][j] = matrix[j][i];
matrix[j][i] = temp;
}
}
// Step 2: Reverse each row
for (int i = 0; i < n; i++) {
int left = 0, right = n - 1;
while (left < right) {
int temp = matrix[i][left];
matrix[i][left] = matrix[i][right];
matrix[i][right] = temp;
left++;
right--;
}
}
// Print rotated matrix
for (int[] row : matrix) {
for (int val : row) {
System.out.print(val + " ");
}
System.out.println();
}
}
}
`
`
Q #22) Given a non-negative integer num, repeatedly add all its digits until the result has only one digit.
Q #23) Given an integer, write a function to determine if it is a power of two.
public boolean isPowerOfTwo(int n) {
return (n > 0) && ((n & (n - 1)) == 0);
}
Q #24) Given an array nums, write a function to move all 0's to the end of it while maintaining the relative order of the non-zero elements.
Q #25) Given an array nums of n integers where nums[i] is in the range [1, n], return an array of all the integers in the range [1, n] that do not appear in nums.
import java.util.*;
public class FindMissingNumbers {
public static void main(String[] args) {
int[] nums = {4,3,2,7,8,2,3,1};
List<Integer> result = new ArrayList<>();
// Step 1: Mark visited indices negative
for (int i = 0; i < nums.length; i++) {
int index = Math.abs(nums[i]) - 1;
if (nums[index] > 0) {
nums[index] = -nums[index];
}
}
// Step 2: Find positive indices
for (int i = 0; i < nums.length; i++) {
if (nums[i] > 0) {
result.add(i + 1);
}
}
System.out.println(result);
}
}
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