BST vs Hash Map Trade-offs for Ordered Data in DSA Java - Interactive Comparison Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to insert a key into a Binary Search Tree (BST).

DSA Java

if (root == null) {
    root = new TreeNode([1]);
}

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

Avalue

Bkey

Cnull

Droot

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'value' instead of 'key' when creating the node.

The BST node is created with the key value when the root is null.

2fill in blank

medium

Complete the code to check if a key exists in a HashMap.

DSA Java

if (map.[1](key)) {
    return true;
}

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

AcontainsKey

BcontainsValue

Cget

Dput

Attempts:

3 left

💡 Hint

Common Mistakes

Using containsValue instead of containsKey.

HashMap's containsKey method checks if the key exists.

3fill in blank

hard

Fix the error in the BST search code to compare keys correctly.

DSA Java

if (key.[1](root.key) < 0) {
    return search(root.left, key);
}

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

AtoString

BhashCode

Cequals

DcompareTo

Attempts:

3 left

💡 Hint

Common Mistakes

Using equals instead of compareTo for ordering.

Use compareTo to compare keys in BST for ordering.

4fill in blank

hard

Fill both blanks to create a TreeMap and insert a key-value pair.

DSA Java

Map<String, Integer> map = new [1]<>();
map.[2]("apple", 5);

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

ATreeMap

Bput

CHashMap

Dadd

Attempts:

3 left

💡 Hint

Common Mistakes

Using HashMap instead of TreeMap for ordered data.

TreeMap maintains order; put inserts key-value pairs.

5fill in blank

hard

Fill all three blanks to iterate over a TreeMap and print keys in order.

DSA Java

for ([1] key : map.[2]()) {
    System.out.println(key + ": " + map.[3](key));
}

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

AString

BkeySet

Cget

DentrySet

Attempts:

3 left

💡 Hint

Common Mistakes

Using entrySet() but iterating as keys only.

Use String for keys, keySet() to get keys in order, and get() to access values.

Practice

1. Which data structure is best suited for storing data when you need to maintain the order of elements and perform fast range queries?

easy

A. Binary Search Tree (BST)

B. Hash Map

C. Stack

D. Queue

Solution

1. Step 1: Understand the need for order and range queries

   Maintaining order and performing range queries requires a data structure that keeps elements sorted.

2. Step 2: Compare BST and Hash Map for order maintenance

   BST keeps elements in sorted order, allowing efficient range queries. Hash Map does not maintain order.

3. Final Answer:

   Binary Search Tree (BST) -> Option A

4. Quick Check:

   Order + range queries = BST [OK]

Hint: Order and range queries need BST, not Hash Map [OK]

Common Mistakes:

• Thinking Hash Map maintains order
• Confusing Stack or Queue with ordered data structures

2. Which of the following is the correct way to declare a HashMap in Java that maps Integer keys to String values?

easy

A. HashMap map = new HashMap();

B. HashMap map = new HashMap<>();

C. HashMap map = new HashMap();

D. HashMap map = new HashMap<>();

Solution

1. Step 1: Check generic type syntax in Java

   Java generics require wrapper classes like Integer, not primitive int.

2. Step 2: Verify constructor and type parameters

   Correct syntax uses diamond operator <> with matching types on left and right sides.

3. Final Answer:

   HashMap map = new HashMap<>(); -> Option D

4. Quick Check:

   Correct generic syntax = HashMap map = new HashMap<>(); [OK]

Hint: Use wrapper types and diamond operator for HashMap [OK]

Common Mistakes:

• Using primitive int instead of Integer
• Mismatching generic types in constructor
• Omitting generic types completely

3. Consider the following Java code using a TreeMap and a HashMap:

import java.util.*;
public class Test {
  public static void main(String[] args) {
    Map treeMap = new TreeMap<>();
    Map hashMap = new HashMap<>();
    treeMap.put(3, "C");
    treeMap.put(1, "A");
    treeMap.put(2, "B");
    hashMap.put(3, "C");
    hashMap.put(1, "A");
    hashMap.put(2, "B");
    System.out.println(treeMap.keySet());
    System.out.println(hashMap.keySet());
  }
}

What is the output of this program?

medium

A. [1, 2, 3]
[3, 1, 2]

B. [3, 1, 2]
[1, 2, 3]

C. [1, 2, 3]
[1, 2, 3]

D. [3, 2, 1]
[3, 2, 1]

Solution

1. Step 1: Understand TreeMap key ordering

   TreeMap stores keys in natural ascending order, so keys are [1, 2, 3].

2. Step 2: Understand HashMap key ordering

   HashMap does not guarantee order; keys print in insertion order or arbitrary order, here [3, 1, 2].

3. Final Answer:

   [1, 2, 3] [3, 1, 2] -> Option A

4. Quick Check:

   TreeMap sorted, HashMap unordered = [1, 2, 3] [3, 1, 2] [OK]

Hint: TreeMap keys sorted; HashMap keys unordered [OK]

Common Mistakes:

• Assuming HashMap keys are sorted
• Confusing insertion order with sorted order

4. The following Java code attempts to use a HashMap to maintain sorted keys. Identify the error:

import java.util.*;
public class Demo {
  public static void main(String[] args) {
    HashMap map = new HashMap<>();
    map.put(2, "B");
    map.put(1, "A");
    map.put(3, "C");
    for (Integer key : map.keySet()) {
      System.out.print(key + " ");
    }
  }
}

medium

A. Keys are printed in ascending order

B. Syntax error in HashMap declaration

C. HashMap does not maintain sorted order of keys

D. NullPointerException at runtime

Solution

1. Step 1: Analyze HashMap behavior for key order

   HashMap does not guarantee any order of keys; iteration order is unpredictable.

2. Step 2: Identify the error in expectation

   The code expects sorted keys but uses HashMap, which cannot maintain sorted order.

3. Final Answer:

   HashMap does not maintain sorted order of keys -> Option C

4. Quick Check:

   HashMap unordered keys = HashMap does not maintain sorted order of keys [OK]

Hint: HashMap keys unordered; use TreeMap for sorting [OK]

Common Mistakes:

• Expecting sorted keys from HashMap
• Confusing HashMap with TreeMap
• Thinking iteration order is insertion order

5. You need to store a large dataset of user IDs and their scores. You want to quickly find users by ID and also retrieve users in ascending order of their IDs. Which data structure combination is best to optimize both fast lookup and ordered traversal?

hard

A. Use a HashMap for fast lookup and a separate sorted list for ordered traversal

B. Use a TreeMap which provides both fast lookup and maintains keys in sorted order

C. Use only a HashMap since it provides both fast lookup and ordered traversal

D. Use a LinkedList for ordered traversal and a HashMap for lookup

Solution

1. Step 1: Understand requirements for fast lookup and ordered traversal

   Fast lookup by ID requires O(1) or O(log n) access; ordered traversal requires sorted keys.

2. Step 2: Evaluate data structure options

   HashMap offers O(1) lookup but no order; TreeMap offers O(log n) lookup and maintains sorted keys.

3. Step 3: Choose best combined solution

   TreeMap alone satisfies both requirements efficiently without extra data structures.

4. Final Answer:

   Use a TreeMap which provides both fast lookup and maintains keys in sorted order -> Option B

5. Quick Check:

   TreeMap = fast lookup + sorted keys [OK]

Hint: TreeMap combines lookup speed and sorted keys [OK]

Common Mistakes:

• Using HashMap alone expecting sorted keys
• Using separate structures causing extra overhead
• Choosing LinkedList which is slow for lookup