Min Heap vs Max Heap When to Use Which in DSA Java - Interactive Comparison Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a min heap using Java's PriorityQueue.

DSA Java

PriorityQueue<Integer> minHeap = new PriorityQueue<>(); // Creates a [1] heap by default

Drag options to blanks, or click blank then click option'

Amin

Bmax

Csorted

Drandom

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to pass a comparator for min heap is unnecessary.

Using a max heap comparator by mistake.

In Java, PriorityQueue with no arguments creates a min heap by default — the smallest element is always at the top.

2fill in blank

medium

Complete the code to create a max heap using Java's PriorityQueue with a comparator.

DSA Java

PriorityQueue<Integer> maxHeap = new PriorityQueue<>([1]);

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Anull

BComparator.naturalOrder()

CComparator.reverseOrder()

DComparator.comparingInt(x -> x)

Attempts:

3 left

💡 Hint

Common Mistakes

Using Comparator.naturalOrder() creates a min heap instead.

Passing null causes default min heap behavior.

To create a max heap, use Comparator.reverseOrder() to reverse the natural ascending order.

3fill in blank

hard

Fix the error in this code snippet that tries to add elements to a max heap.

DSA Java

PriorityQueue<Integer> maxHeap = new PriorityQueue<>([1]);
maxHeap.add(10);
maxHeap.add(20);
maxHeap.add(15);

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AComparator.reverseOrder()

BComparator.naturalOrder()

Cnull

DComparator.comparingInt(x -> -x)

Attempts:

3 left

💡 Hint

Common Mistakes

Using Comparator.naturalOrder() results in min heap behavior.

Passing null defaults to min heap.

Using Comparator.reverseOrder() correctly creates a max heap; other options create min heap or cause errors.

4fill in blank

hard

Fill both blanks to create a min heap and add an element to it.

DSA Java

PriorityQueue<Integer> heap = new PriorityQueue<>(); // [1] heap
heap.[2](5);

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Amin

Badd

Coffer

Dremove

Attempts:

3 left

💡 Hint

Common Mistakes

Passing a comparator for min heap unnecessarily.

Using remove() instead of add() to insert.

No-argument PriorityQueue creates a min heap. Use add() to insert an element.

5fill in blank

hard

Fill all three blanks to create a max heap, add an element, and remove the top element.

DSA Java

PriorityQueue<Integer> heap = new PriorityQueue<>([1]);
heap.[2](10);
int top = heap.[3]();

Drag options to blanks, or click blank then click option'

AComparator.reverseOrder()

Badd

Cpoll

Dremove

Attempts:

3 left

💡 Hint

Common Mistakes

Using remove() instead of poll() can throw exceptions if empty.

Using naturalOrder() creates min heap.

Comparator.reverseOrder() creates max heap; add() inserts; poll() removes top safely.

Practice

1. What is the main difference between a Min Heap and a Max Heap?

easy

A. Min Heap always keeps the smallest element at the root, Max Heap keeps the largest at the root

B. Min Heap stores elements in ascending order, Max Heap stores elements in descending order

C. Min Heap is used only for sorting, Max Heap is used only for searching

D. Min Heap and Max Heap have the same structure but different sizes

Solution

1. Step 1: Understand heap root property

   Min Heap keeps the smallest element at the root node, Max Heap keeps the largest element at the root node.

2. Step 2: Compare heap ordering

   Min Heap ensures parent nodes are smaller than children, Max Heap ensures parent nodes are larger than children.

3. Final Answer:

   Min Heap always keeps the smallest element at the root, Max Heap keeps the largest at the root -> Option A

4. Quick Check:

   Root element defines Min or Max Heap [OK]

Hint: Root holds smallest in Min Heap, largest in Max Heap [OK]

Common Mistakes:

• Confusing heap order with sorted order
• Thinking heaps store elements fully sorted
• Assuming Min and Max Heap differ in size

2. Which Java class correctly creates a Min Heap using the PriorityQueue?

easy

A. PriorityQueue<Integer> minHeap = new PriorityQueue<>();

B. PriorityQueue<Integer> minHeap = new PriorityQueue<>(Collections.reverseOrder());

C. PriorityQueue<Integer> minHeap = new PriorityQueue<>(Comparator.naturalOrder().reversed());

D. PriorityQueue<Integer> minHeap = new PriorityQueue<>(Comparator.reverseOrder());

Solution

1. Step 1: Check default PriorityQueue behavior

   Java's PriorityQueue by default acts as a Min Heap, so no comparator is needed for Min Heap.

2. Step 2: Analyze options

   PriorityQueue<Integer> minHeap = new PriorityQueue<>(); (PriorityQueue<Integer> minHeap = new PriorityQueue<>();) uses default constructor, which creates a Min Heap. Options A, C, and D use reverse order comparators, creating Max Heap.

3. Final Answer:

   PriorityQueue<Integer> minHeap = new PriorityQueue<>(); -> Option A

4. Quick Check:

   Default PriorityQueue = Min Heap [OK]

Hint: Default PriorityQueue is Min Heap in Java [OK]

Common Mistakes:

• Using reverseOrder() for Min Heap
• Confusing naturalOrder() with reverseOrder()
• Adding unnecessary comparator for Min Heap

3. What will be the output of the following Java code snippet?

PriorityQueue<Integer> maxHeap = new PriorityQueue<>(Collections.reverseOrder());
maxHeap.add(10);
maxHeap.add(5);
maxHeap.add(20);
System.out.println(maxHeap.poll());

medium

A. 10

B. 20

C. 5

D. Runtime error

Solution

1. Step 1: Understand heap type from constructor

   Using Collections.reverseOrder() creates a Max Heap, so the largest element is at the root.

2. Step 2: Analyze inserted elements and poll()

   Elements 10, 5, 20 are added. The largest is 20, so poll() removes and returns 20.

3. Final Answer:

   20 -> Option B

4. Quick Check:

   Max Heap root = largest element = 20 [OK]

Hint: poll() returns root: largest in Max Heap [OK]

Common Mistakes:

• Assuming poll() returns smallest element
• Confusing default PriorityQueue as Max Heap
• Expecting sorted output instead of root element

4. Identify the error in this code snippet that tries to create a Max Heap:

PriorityQueue<Integer> maxHeap = new PriorityQueue<>();
maxHeap.add(15);
maxHeap.add(30);
maxHeap.add(10);
System.out.println(maxHeap.poll());

medium

A. No error; code creates a Max Heap correctly

B. Missing import for Collections class

C. poll() method does not exist for PriorityQueue

D. PriorityQueue default is Min Heap, so poll() returns smallest, not largest

Solution

1. Step 1: Check PriorityQueue default behavior

   PriorityQueue without comparator is a Min Heap, so poll() returns smallest element.

2. Step 2: Analyze expected vs actual output

   Code expects Max Heap but poll() returns 10 (smallest), not 30 (largest), so behavior is incorrect for Max Heap.

3. Final Answer:

   PriorityQueue default is Min Heap, so poll() returns smallest, not largest -> Option D

4. Quick Check:

   Default PriorityQueue ≠ Max Heap [OK]

Hint: Default PriorityQueue is Min Heap, not Max Heap [OK]

Common Mistakes:

• Assuming PriorityQueue default is Max Heap
• Forgetting to use comparator for Max Heap
• Confusing poll() with peek() behavior

5. You need to efficiently find the median of a stream of numbers in Java. Which heap combination is best to use and why?

hard

A. Use one Min Heap only, because it keeps smallest elements at root

B. Use one Max Heap only, because it keeps largest elements at root

C. Use two heaps: Max Heap for lower half, Min Heap for upper half to balance and find median

D. Use two Min Heaps to store all elements and find median quickly

Solution

1. Step 1: Understand median finding strategy

   Median requires balanced halves: lower half and upper half of numbers.

2. Step 2: Choose appropriate heaps for halves

   Max Heap stores lower half (largest at root), Min Heap stores upper half (smallest at root). Balancing these heaps allows median calculation efficiently.

3. Final Answer:

   Use two heaps: Max Heap for lower half, Min Heap for upper half to balance and find median -> Option C

4. Quick Check:

   Two heaps balance halves for median [OK]

Hint: Two heaps balance halves: Max for low, Min for high [OK]

Common Mistakes:

• Using only one heap for median
• Using two Min Heaps instead of Min and Max
• Not balancing heaps after insertions