Crack the top 40 Java coding interview questions

Java remains a popular language across the world. Especially in financial fields, Java’s scalability and efficiency keep it in-demand with interviewers in a variety of famous companies like Goldman Sachs, eBay, and Google.

Today, we’ll help you prepare for your upcoming coding interview at these and other popular eCommerce companies by reviewing the top 40 Java questions asked by interviewers. By the end, you’ll have the experience to walk into any Java interview with confidence.

Let’s get started!

Here are the types of questions we’ll cover today:

• Big O Notation
• Data Structures
• Recursion
• Dynamic Programming
• Mixed Subject Questions
• Wrapping up and next steps

1. Nested Loop with Addition

Find the Big O complexity of the following code:

class NestedLoop {
  public static void main(String[] args) {
    int n = 10; // 1 step --> Note: n can be anything. This is just an example
    int sum = 0; // 1 step
    double pie = 3.14; // 1 step
    
    for (int var = 0; var < n; var = var + 3) { // n/3 steps
      System.out.println("Pie: " + pie); // n/3 steps
      for (int j = 0; j < n; j = j + 2) {  // (n/3 * n/2) steps
        sum++;   // (n/3 * n/2) steps
        System.out.println("Sum = " + sum);  // (n/3 * n/2) steps
     }
   }
  }
}

Solution Explanation:

The first for loop on line 7 can be broken down into 3 parts:

• initialization
• comparison
• incrementation

Since the initialization (int var = 0) only happens once in the entire program, it takes 1 unit of time. The comparison (var < n) gets executed $\frac{n}{3} + 1$ times and the increment runs $\frac{n}{3}$ times.

Similarly, (int j = 0) runs $\frac{n}{3} times , the comparison (j < n) runs $n/3 \times$ $\frac{n}{2}$ $+ 1$.

The test case runs once more than the whole loop where boundary check fails!

The increment (j = j + 2) gets executed $\frac{n}{2}$ times for each iteration of the outer loop–which makes it run a total of $\frac{n}{3} \times \frac{n}{2} = \frac{n^2}{6}$ times.

Run time complexity:

$\frac{(15+5n+2n^2)} {3}$

To find Big O, we:

• Drop leading constants
• Drop lower order terms

Therefore, Big O Complexity: $O(n^2)$

2. Nested Loop with Multiplication

Find the Big O complexity of the following code segment:

class NestedLoop {
	public static void main(String[] args) {
		int n = 10; // O(time complexity of the called function)
		int sum = 0; //O(1)
		double pie = 3.14; //O(1)
		int var = 1;

	
		while(var < n) {  // O(log n)
			System.out.println("Pie: " + pie); // O(log n)
			
			for (int j = 0; j < var; j++) {  // 2n
				sum++;  //  (2n-1)
			}
			var *= 2; // O(log n)
		} //end of while loop
		
		System.out.println("Sum: " + sum); //O(1)
	} //end of main
} //end of class

Solution Explanation:

A loop statement that multiplies/divides the loop variable by a constant takes $\log_k(n) time because the loop runs that many times. In the outer loop, the loop variable is multiplied by 2 in each iteration. Therefore, the outer loop runs $O(log_2 \space n)$ times.

The inner loop runs var times, not n times. The value of var is 1 in the first iteration, then 2, then 4, then 8, and so on until 2^ksuch that `2^k < 2. So when we sum the var values for all the iterations of the outer loop, the inner loop runs $2^k$ times. We’ll use a geometric series to figure out this value. To make this calculation simpler, let’s assume that $2^k = n$:

$2^{k+1} - 1$

​$2^k2^1 - 1$

Substituting n for $2^k$ we get:

= $2n-1$

So it appears that the inner loop runs a total of $2n-1$ times (Considering all the iterations of the outer loop), but remember that we assumed that $n = 2^k$ when $n>2^k$ so, in actuality, the inner loop runs less than $2n-1$ times, but we can consider this to be the upper bound.

Runtime Complexity: $4+4log_ 2(n)+6n$

Big O: $O(n)$

3. Nested Loop with Multiplication (Advanced)

Find the Big O Complexity of the following code segment:

 class NestedLoop {
 	public static void main(String[] args) {
     int n = 10;   
		int sum = 0; 
 		double pie = 3.14; 

		for (int var = 0; var < n; var++) {   
      int j = 1;  
			System.out.println("Pie: " + pie);
			while(j < var) { 
        sum += 1;  
        j *= 2;  
      }
    } //end of for loop
    System.out.println("Sum: " + sum); 
  } //end of main
 } //end of class

Solution Explanation:

In the main function, the outer loop is $O(n)$ as it iterates n times. The inner while loop iterates var times, which is always less than n and the inner loop counter variable is doubled each time. Therefore we can say that it is $O(log_2(n))$. Thus, the total time complexity of the program given above becomes:

$O(nlog_2(n))$

The running time complexity mentioned above is a loose bound. To evaluate a tighter bound on the running time of the program given above, let’s look at the inner loop once again.

The inner loop depends upon j which is less than var and is multiplied by 2 in each iteration. This means that the complexity of the inner loop is $O(log_2(\text{var}))$. But, the value of var at each iteration, of the outer loop, is different. The total complexity of the inner loop in terms of n can be calculated as such:

$n \times log_2(var)$

$\Rightarrow\sum_{var=1}^{n} log_2 (var)$

$\Rightarrow log_2 (1) +... + log_2 (n-1) + log_2 (n)$

Further, we know:

$log(a) + log(b) = log(ab)$

Therefore, the above expression becomes:

$\Rightarrow log_2 (1 \times 2 \times ... \times (n-1) \times(n) )$

$\Rightarrow log_2 (n!)$

Thus, the total time complexity of the inner-loop (considering the outer-loop) is $O(log_2(n!))$.

Time complexity: $7+4n+3log_​2 (n!)$

Big O: $O(log_2(n!))$.

This is a tight upper-bound to the growth function of this program. And as mentioned above, the loose upper bound is $O(nlog_2(n))$.

4. Conditional Loop

What is the Big O complexity of the following code segment:

    void complexMethod(int[] array) {
        int n = array.length;
        int runFor = Math.pow(-1, n) * Math.pow(n, 2);
        for (int i = 0; i < runFor; i++) {
            System.out.println("Find how complex I am ?")
        }
    }

Solution Explanation:

A common mistake with this question is to say that the complexity is $O(n)$. This is incorrect because the worst case only happens for even sizes of the input array.

The loop doesn’t run when the size of the array is an odd number.

Next, the loop runs a quadratic number of times the length of the array when the length is even.

The bigger the array size and provided it is an even number, the single loop runs as if it were a nesting of two loops.

Therefore, the complexity of this snippet of code is $O(n^2)$.

5. Heavily nested loops

Find the Big O complexity of the following code segment:

    void someMethod(int n, int m) {
 
       for (int j = 0; j < n; j++) {
          for (int i = 0; i < 3; i++) {   
             for (int i = 0; i < n; i++) {
                 System.out.println("I have 3 loops");
             }
          } 
       }
    }

Solution Breakdown:

The best way to solve this problem is to unroll the second loop. Here’s what this code would look like unrolled:

    void someMethod(int n) {
 
       for (int j = 0; j < n; j++) {   
          for (int i = 0; i < n; i++) {
              System.out.println("I have 3 loops");
          }
          for (int i = 0; i < n; i++) {
              System.out.println("I have 3 loops");
          }
          for (int i = 0; i < n; i++) {
              System.out.println("I have 3 loops");
          }
       }
    }

From the unrolling, it is evident that the three inner loops contribute $n + n + n = 3n = O(n)$ and the outermost loop runs for n too, therefore the overall complexity is $O(n^2)$.

Data Structures

6. Array of Products of All Elements Except Itself

Problem Statement:

“Implement the int[] findProduct(int[] arr) method which will modify arr in such a way that in the output, each index i will contain the product of all elements present in arr except the element stored on that index i.”

Solution and Explanation:

Time Complexity: $O(n)$

The algorithm for this solution is to first create a new array with products of all elements to the left of each element, as done on lines 17-20.

Then on lines 27-30, multiply each element in that array to the product of all the elements to the right of the array by traversing it in reverse.

7. Remove Even Integers from an Array

Problem Statement:

“In this problem, you have to implement the int [] removeEven(int[] arr) method, which removes all the even elements from the array and returns an updated array. The final array should contain only odd elements with the array reduced in size accordingly.”

Solution and Explanation:

Time Complexity: $O(n)$

On lines 6 - 9 finds the number of odd elements in the given array by iterating over it and incrementing a count variable if an element is odd.

Next, on lines 11 - 13 we initialize an array with a size oddElements, and store all the odd numbers in it.

Finally, on lines 16 - 19 we fill the array result with all elements from oddElements to return as our solution.

8. Remove duplicates from a Linked List

Problem Statement:

“Given a string that contains duplicate occurrences of characters, remove every character that has already appeared previously in the list.”

Solution and Explanation:

Time Complexity: $O(n)$

In this solution, we’ll keep two pointers, one for the current reading position and one for the current writing position.

Whenever we encounter the first occurrence of a character, we add it to the HashSet. Any character already existing in the HashSet is skipped on any subsequent occurrence.

Below is an overview of the algorithm in pseudocode:

read_pos = 0
write_pos = 0
 
for each character 'c' in str
  if 'c' not found in hashset
    add 'c' to hashset
    str[write_pos] = str[read_pos]
    write_pos++
  read_pos++

9. Longest Palindromic Subsequence

Problem Statement:

"Given a sequence, find the length of its Longest Palindromic Subsequence (LPS). In a palindromic subsequence, elements read the same backward and forward.

A subsequence is a sequence that can be derived from another sequence by deleting some or no elements without changing the order of the remaining elements."

Input: "abdbca"
Output: 5
Explanation: LPS is "abdba".

Solution and Explanation:

Time Complexity: $O(2^n)$

A basic brute-force solution could be to try all the subsequences of the given sequence. We can start processing from the beginning and the end of the sequence. So at any step, we have two options:

If the element at the beginning and the end are the same, we increment our count by two and make a recursive call for the remaining sequence.

We will skip the element either from the beginning or the end to make two recursive calls for the remaining subsequence.

If option one applies then it will give us the length of LPS; otherwise, the length of LPS will be the maximum number returned by the two recursive calls from the second option.

Solution 2: Dynamic Programming:

Time Complexity: $O(n^2)$

We can use an array to store the already solved subproblems.

The two changing values to our recursive function are the two indexes, startIndex and endIndex. Therefore, we can store the results of all the subproblems in a two-dimensional array. (Another alternative could be to use a hash-table whose key would be a string (startIndex + “|” + endIndex))

10. Find the Length of a Linked List

Problem Statement:

“Implement the method int length(), which will count the number of nodes in a linked list.”

Solution and Explanation:

Time Complexity: $O(n)$

The trick is to iterate through the list and keep count of how many nodes you’ve visited. This count is held in a variable, count, that is returned when the end of the list is reached.

11. Detect Loop in a Linked List

Problem Statement:

"In this problem, you have to implement the public static boolean detectLoop(SinglyLinkedList list) method, which will take a Singly linked list as input and find if there is a loop present in the list. "

A loop in a linked list occurs if any node contains a reference to any previous node, then a loop will be formed.

You’re provided the following class to use in your solution:

Solution and Explanation:

Time Complexity: $O(n)$

This is the most optimized method to find out the loop in the LinkedList and uses Floyd’s Cycle Detection Algorithm. We start traversing the LinkedList using two pointers called slow and fast.

Move slow by one (line 9) and fast by two (line 10). If these pointers meet at the same node, then there is a loop. If these pointers do not meet, then LinkedList doesn’t have a loop.

12. Implement Two Stacks using one Array

Problem Statement:

“Create class TwoStacks<V> that uses a single array to generate two stacks. Your class will have the following methods:”

void push1(V value)
void push2(V value)
public V pop1()
public V pop2()

push1(input)

• Input: an integer
• Output: inserts the given value in the first stack i.e. stack1

push2(input)

• Input: an integer
• Output: inserts the given value in the second stack i.e stack2

pop1()

• Output: returns and remove the top value of stack1

pop2()

• Output: returns and remove the top value of stack2

You’re supplied the following code to begin your solution:

Solution and Explanation:

Time Complexity: $O(1)$

This implementation is space-efficient as it utilizes all of the available space. It doesn’t cause an overflow if there is any space available in the array.

The tops of the two stacks are the two extreme ends of the array. The first stack starts from the first element at index 0, and the second starts from the last element.

The first element in stack2 is pushed at index (maxSize-1). Both stacks grow (or shrink) in the opposite direction.

To check for overflow, all we need do is check for space between the top elements of both stacks, as reflected in the code.

13. Level Averages in a Binary Tree

Problem Statement:

“Given a binary tree, populate an array to represent the averages of all of its levels.”

Solution and Explanation:

Time Complexity: $O(n)$

This problem follows the Binary Tree Level Order Traversal pattern. We can follow the same BFS approach.

The only difference will be that instead of keeping track of all nodes of a level, we will only track the running sum of the values of all nodes in each level.

In the end, we will append the average of the current level to the result array.

14. Binary Tree Path Sum

Problem Statement:

"Given a binary tree and a number S, find if the tree has a path from root-to-leaf such that the sum of all the node values of that path equals S. "

Solution and Explanation:

Time Complexity: $O(n)$

As we are trying to search for a root-to-leaf path, we can use the Depth First Search (DFS) technique to solve this problem.

To recursively traverse a binary tree in a DFS fashion, we can start from the root and at every step, make two recursive calls one for the left and one for the right child.

Here are the steps for our Binary Tree Path Sum problem:

1. Start DFS with the root of the tree.
2. If the current node is not a leaf node, do two things:
   • Subtract the value of the current node from the given number to get a new sum => S = S - node.value
   • Make two recursive calls for both the children of the current node with the new number calculated in the previous step.
3. At every step, see if the current node being visited is a leaf node and if its value is equal to the given number S. If both these conditions are true, we have found the required root-to-leaf path, therefore return true.
4. If the current node is a leaf but its value is not equal to the given number S, return false.

15. Find All of the Words in a Trie

Problem Statement:

“Implement the findWords() function that returns all of the words stored in a Trie in alphabetically sorted order. There are 9 words total in the given keys array, so we just need to iterate the Trie and return all of the words present in it.”

Solution and Explanation:

Time Complexity: $O(n)$

The findWords(root) function contains a result ArrayList which will contain all the words in the trie. word is a character array in which node characters are added one by one to keep track of all the alphabets in the same recursive call.

getWords() is our recursive function which begins from the root and traverses every node. Whenever a node is the end of a word, temp (containing the character array) is converted into a string and inserted into result.

Since word cannot be reset before recording every new word, we simply update the values at each index using level.

Recursion

16. Find the Greatest Common Divisor

Problem Statement:

“Write a recursive method that computes the Greatest Common Divisor of two integers.”

The GCD of two integers is the largest integer that can fully divide both numbers without a remainder.

• The method should take two integers as input.
• Their GCD is to be computed, as input.
• The method should return the GCD of the two integers as output.
• The method should be recursive.

Solution and Explanation:

Recursive Method

The return type of this method is an integer because GCD of two numbers is always an integer.

The method takes two integer input arguments num1 and num2.

Base Case

The base case of the method is defined in the if block, from line 6 - 8.

When num2 is equal to num1, the method returns num1.

Recursive Case

If the base case does not compute to be true, the method checks the second if condition on line 10, to be true i.e if num1 > num2.

If it computes to true, a recursive call is made with parameters num1-num2 and num2 respectively.

If the base case and the if condition does not compute to true, the method automatically goes to the else statement on line 14, where num2 > num1 and a recursive call is made with parameters num1 and num2-num1 respectively.

In the case above the recursive calls will be as follows:

First recursive call will be for num2 > num1 and will be gcd(56-42,42).

Second recursive call will be for num1 > num2 and will be gcd(14,42-14).

Third recursive call will be for num1 > num2 and will be gcd(14,28-14).

After the third recursive call, the num1==num2 condition computes to true.

So, the base case is reached and the method returns num1, which is equal to $14$.

17. Check for Prime Number

Problem Statement:

“Write a recursive method, named isPrime() that checks if a number is prime or not.”

• The method should take two integers as input.
• The method should return Boolean, meaning it will return true if the number is prime or return false if the number is not prime.
• The method should be recursive.

Solution and Explanation:

Recursive Method

The return type of this method is Boolean since we want to check if the number is prime or not and it takes the two integers, num and i, as input parameters.

num is the number to be tested and i is the iterator and has the initial value of num/2 when the method is called in the main code.

i has the initial value of num/2 because all the factors of a number are less than equal to the half of that number. We iterate through all the numbers less than or equal to i to check if num has a factor. We use the % operator to check for the factors of num.

Base Case

We have defined the base cases for the method in the if conditions.

In the first if condition, line 6-8, when num is less than or equal to $2$, the method returns num since $0$ and $1$ are not prime numbers.

In the second if condition, lines 10-12, when i is equal to $1$, the method returns true. We will need to understand the recursive part of the method to understand this base case.

In the third if condition, lines 14-16, if the number has a divisor other than $1$, the method returns false. To check for this condition, we use the % operator to compute modulo of num with the iterator i.

Recursive Case

When the if condition for any of the base cases does not compute to be true, the method enters the else block, lines 18-20, where it makes a recursive call.

In the recursive call, the isPrime() method is called with the first parameter, num, and the second parameter, i-1.

In this case, the recursive calls will be as follows:

• The first recursive call will be isPrime(13,5).

• The second recursive call will be isPrime(13,4).

• The third recursive call will be isPrime(13,3).

• The fourth recursive call will be isPrime(13,2).

• The fifth recursive call will be isPrime(13,1) which is when the second base case condition will be reached since i==1.

The method will return true since num is divisible by $1$ only.

18. Recursive Insert into Binary Search Tree

Problem Statement:

“Create a recursive method that inserts an element at it’s appropriate position in a binary search tree.”

Solution and Explanation:

Recursive Method

The return type of the insert() function is boolean and takes one integer type of input parameter value which depicts the value of the node.

This method calls the actual recursive method recursive_insert that takes two input parameters. The first is currentNode of type Node. The second parameter value is the integer to be inserted in the BST. We will discuss the recursive_insert method below.

Base Case

We have defined a base case for the method in first if condition on lines 19 - 21.

If the value of the currentNode is null, meaning there is an available space for the child node to be inserted, a new Node() is created with the value.

Recursive Case

If the base case condition is not met and the value to be inserted is less than the value of currentNode.getData(), the function enters the second if condition, line 25, where it makes a recursive call.

In this process, the recursive_insert method is recursive. The first parameter is currentNode.getLeftChild() and the second parameter is value.

If the method has not reached the base and the inserted value is greater than the value of currentNode.getData(), the function enters the other else if condition, in line 26-28, where it makes a recursive call.

In this else if condition, the recursive_insert method is called. The first parameter is currentNode.getRightChild() and the second parameter is value.

If all the above conditions are not met, we reach the else condition from lines 29 to 31 which returns the currentNode since the defined value already exists.

Line 34 returns the currentNode after being inserted into its position.

Subsequent recursive calls will continually be made until the first parameter equals null, which ensures the availability of a position for the new node. The new node is then added to its corresponding position.

Dynamic Programming

19. Remove duplicates from a Linked List

Problem Statement:

“Given a number array to represent different coin denominations and a total amount T, find all the different ways to make change for T with the given coin denominations. We can assume an infinite supply of coins, therefore, each coin can be chosen multiple times.”

Solution and Explanation:

A basic brute-force solution could be to try all combinations of the given coins to select the ones that give a total sum of T. Here is what that would look like in pseudocode:

for each coin 'c' 
  create a new set which includes one quantity of coin 'c' if it does not exceed 'T', and 
     recursively call to process all coins 
  create a new set without coin 'c', and recursively call to process the remaining coins 
return the count of sets who have a sum equal to 'T'

We can instead use Top-Down Dynamic Programming with Memoization to optimize our solution.

We can use memoization to overcome the overlapping sub-problems. We will be using a two-dimensional array to store the results of solved sub-problems. We need to store results for every coin combination and for every possible sum.

Time Complexity: $O(C \times T)$

20. Minimum jumps to reach the end

Problem Statement:

“Given an array of positive numbers, where each element represents the max number of jumps that can be made forward from that element, write a program to find the minimum number of jumps needed to reach the end of the array (starting from the first element). If an element is 0, then we cannot move through that element.”

// Example input and output

Input = {2,1,1,1,4}
Output = 3
Explanation: Starting from index '0', we can reach the last index through: 0->2->3->4

Solution and Explanation:

We will start with the 0th index and try all options. So, if the value at the current index is p, we will try every jump in the range (1 to p) from that index. After taking a jump, we recursively try all options from that index.

However, we can optimize this solution with a bottom up approach.

Let’s try to populate our dp[] array from the above solution, working in the bottom-up fashion. As we saw in the above code, we were trying to find the minimum jumps needed to reach every index (if it is within the range) from the current index. We can use this fact to populate our array.

As we know, every index within the range of the current index can be reached in one jump. Therefore, we can say that we can reach every index (within the range of current index) in:

    'jumps to reach current index' + 1

So, while going through all the indexes, we will take the minimum value between the current jump-count and the jumps needed to reach the current index + 1.

Time Complexity: $O(n^2)$

20 More Mixed Questions

Wrapping up and next steps

Congratulations on finishing those 40 questions! The best way to prepare for coding interviews is the practice you’re doing right now. Soon, you’ll know all the question types you could encounter at your next interview.

To help you prepare for interviews, Educative has created the course Grokking Coding Interview Patterns in Java. You’ll learn the 24 patterns behind every coding interview question, so you can be prepared to answer any problem you might face using Java.

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Happy learning!

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