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30+ Java coding interview questions with solutions covering strings, arrays, patterns, recursion, and data structure problems commonly asked in placements.
1. Reverse a String
Example
public String reverse(String str) {
return new StringBuilder(str).reverse().toString();
}
// Without StringBuilder
public String reverseManual(String str) {
char[] chars = str.toCharArray();
int left = 0, right = chars.length - 1;
while (left < right) {
char temp = chars[left];
chars[left] = chars[right];
chars[right] = temp;
left++; right--;
}
return new String(chars);
}
3. Find Duplicate Characters in String
java example
public Map<Character, Integer> findDuplicates(String str) {
Map<Character, Integer> map = new LinkedHashMap<>();
for (char c : str.toCharArray()) {
map.put(c, map.getOrDefault(c, 0) + 1);
}
map.entrySet().removeIf(e -> e.getValue() == 1);
return map;
}
// "programming" → {r=2, g=2, m=2}Output
"programming" → {r=2, g=2, m=2}4. Fibonacci Series
Example
// Iterative (O(n) time, O(1) space)
public void fibonacci(int n) {
int a = 0, b = 1;
for (int i = 0; i < n; i++) {
System.out.print(a + " ");
int next = a + b;
a = b;
b = next;
}
}
// Output: 0 1 1 2 3 5 8 13 21 34
// Recursive
public int fib(int n) {
if (n <= 1) return n;
return fib(n - 1) + fib(n - 2);
}
Output
0 1 1 2 3 5 8 13 21 34
5. Check if Number is Prime
Example
public boolean isPrime(int n) {
if (n <= 1) return false;
if (n <= 3) return true;
if (n % 2 == 0 || n % 3 == 0) return false;
for (int i = 5; i * i <= n; i += 6) {
if (n % i == 0 || n % (i + 2) == 0) return false;
}
return true;
}
6. Find Second Largest Element in Array
Example
public int secondLargest(int[] arr) {
int first = Integer.MIN_VALUE, second = Integer.MIN_VALUE;
for (int num : arr) {
if (num > first) {
second = first;
first = num;
} else if (num > second && num != first) {
second = num;
}
}
return second;
}
// [12, 35, 1, 10, 34, 1] → 34
7. Two Sum Problem
Example
public 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[]{};
}
// nums=[2,7,11,15], target=9 → [0, 1]
8. Remove Duplicates from Array
Example
public int[] removeDuplicates(int[] arr) {
return Arrays.stream(arr).distinct().toArray();
}
// Without streams (sorted array)
public int removeDuplicatesSorted(int[] arr) {
if (arr.length == 0) return 0;
int j = 0;
for (int i = 1; i < arr.length; i++) {
if (arr[i] != arr[j]) {
arr[++j] = arr[i];
}
}
return j + 1; // new length
}
9. Reverse a Linked List
java example
public ListNode reverseList(ListNode head) {
ListNode prev = null, current = head;
while (current != null) {
ListNode next = current.next;
current.next = prev;
prev = current;
current = next;
}
return prev;
}10. Check Balanced Parentheses
Example
public boolean isBalanced(String s) {
Stack<Character> stack = new Stack<>();
for (char c : s.toCharArray()) {
if (c == '(' || c == '{' || c == '[') {
stack.push(c);
} else {
if (stack.isEmpty()) return false;
char top = stack.pop();
if (c == ')' && top != '(') return false;
if (c == '}' && top != '{') return false;
if (c == ']' && top != '[') return false;
}
}
return stack.isEmpty();
}
// "{[()]}" → true, "{[(])}" → false
Output
"{[()]}" → true, "{[(])}" → false11. Find Missing Number in Array (1 to N)
Example
public int findMissing(int[] arr, int n) {
int expectedSum = n * (n + 1) / 2;
int actualSum = Arrays.stream(arr).sum();
return expectedSum - actualSum;
}
// [1,2,4,5,6], n=6 → 3
12. String Anagram Check
java example
public boolean isAnagram(String s1, String s2) {
if (s1.length() != s2.length()) return false;
char[] a = s1.toLowerCase().toCharArray();
char[] b = s2.toLowerCase().toCharArray();
Arrays.sort(a);
Arrays.sort(b);
return Arrays.equals(a, b);
}
// "listen", "silent" → true13. Binary Search
Example
public int binarySearch(int[] arr, int target) {
int left = 0, right = arr.length - 1;
while (left <= right) {
int mid = left + (right - left) / 2;
if (arr[mid] == target) return mid;
else if (arr[mid] < target) left = mid + 1;
else right = mid - 1;
}
return -1;
}
14. Find First Non-Repeating Character
java example
public char firstNonRepeating(String str) {
LinkedHashMap<Character, Integer> map = new LinkedHashMap<>();
for (char c : str.toCharArray()) {
map.put(c, map.getOrDefault(c, 0) + 1);
}
for (Map.Entry<Character, Integer> entry : map.entrySet()) {
if (entry.getValue() == 1) return entry.getKey();
}
return '\0';
}
// "aabbcdd" → 'c'Output
"aabbcdd" → 'c'
15. Merge Two Sorted Arrays
Example
public int[] merge(int[] a, int[] b) {
int[] result = new int[a.length + b.length];
int i = 0, j = 0, k = 0;
while (i < a.length && j < b.length) {
if (a[i] <= b[j]) result[k++] = a[i++];
else result[k++] = b[j++];
}
while (i < a.length) result[k++] = a[i++];
while (j < b.length) result[k++] = b[j++];
return result;
}
16. Power of Two Check
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Output
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17. Count Vowels and Consonants
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Output
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18. Factorial (Recursive + Iterative)
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Output
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19. GCD using Euclidean Algorithm
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Output
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20. Rotate Array by K Positions
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Output
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21. Maximum Subarray Sum (Kadane's Algorithm)
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Output
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22. Detect Cycle in Linked List (Floyd's Algorithm)
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Output
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23. Stack using Two Queues
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Output
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24. Decimal to Binary Conversion
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Output
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25. Intersection of Two Arrays
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Output
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26. Longest Common Prefix
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Output
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27. Sort Array of 0s, 1s, 2s (Dutch National Flag)
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Output
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28. Matrix Diagonal Sum
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Output
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29. Check Palindrome Linked List
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Output
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30. LRU Cache Implementation
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Output
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Summary
In this chapter, we learned about Coding Interview Questions in detail. Here are the key points:
- Basic introduction to the concept and why it is needed
- Syntax and structure with complete examples
- Internal working and best practices
- Real-world applications and use cases
- Common mistakes and how to avoid them
- How this topic is asked in interviews
To practice these concepts, write and run the code in your IDE and verify the output. Modify each example and experiment so that you deeply understand the concept.
Continue the lesson path
Study next in this module
01Interview QuestionsSpring Framework Interview Questions35+ Spring Framework and Spring Boot interview questions covering IoC, DI, annotations, REST, JPA, Security, and microservices concepts.02Interview QuestionsJDBC Interview Questions30+ JDBC interview questions with detailed answers covering connections, statements, transactions, connection pooling, and best practices.03Interview QuestionsMultithreading Interview Questions35+ Java Multithreading interview questions covering threads, synchronization, locks, executor framework, concurrent collections, and thread communication.04Interview QuestionsCollections Interview Questions35+ Java Collections Framework interview questions covering List, Set, Map, Queue, sorting, concurrent collections, and internal implementations.