Two Sum Problem Classic Hash Solution in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare the dictionary for storing numbers and their indices.

DSA C#

var map = new Dictionary<int, int>[1];

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A{}

B()

C[]

D<>

Attempts:

3 left

💡 Hint

Common Mistakes

Using curly braces {} instead of parentheses () to instantiate the dictionary.

Using angle brackets <> which are for generics, not instantiation.

The dictionary is created with parentheses () to call its constructor.

2fill in blank

medium

Complete the code to check if the complement exists in the dictionary.

DSA C#

if (map.[1](complement))

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AContainsKey

BContainsValue

CContains

DHasKey

Attempts:

3 left

💡 Hint

Common Mistakes

Using ContainsValue which checks values, not keys.

Using Contains which is not a dictionary method.

Use ContainsKey to check if a key exists in the dictionary.

3fill in blank

hard

Fix the error in adding the current number and its index to the dictionary.

DSA C#

map.[1](nums[i], i);

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ASet

BPut

CInsert

DAdd

Attempts:

3 left

💡 Hint

Common Mistakes

Using Put or Set which are not valid dictionary methods in C#.

Using Insert which is for lists, not dictionaries.

The correct method to add a key-value pair to a dictionary in C# is Add.

4fill in blank

hard

Fill both blanks to return the indices of the two numbers that add up to the target.

DSA C#

return new int[] { map[[1]], [2] };

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Acomplement

Bi

Ctarget

Dnums[i]

Attempts:

3 left

💡 Hint

Common Mistakes

Returning the value nums[i] instead of the index i.

Using target instead of complement as dictionary key.

Return the index of the complement from the dictionary and the current index i.

5fill in blank

hard

Fill all three blanks to complete the Two Sum method.

DSA C#

public int[] TwoSum(int[] nums, int [1]) {
    var map = new Dictionary<int, int>[2];
    for (int i = 0; i < nums.Length; i++) {
        int complement = [3] - nums[i];
        if (map.ContainsKey(complement)) {
            return new int[] { map[complement], i };
        }
        map.Add(nums[i], i);
    }
    throw new ArgumentException("No two sum solution");
}

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

Atarget

B()

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong parameter name instead of target.

Forgetting parentheses when creating the dictionary.

Calculating complement incorrectly.

The method parameter is target, the dictionary is instantiated with (), and complement is calculated as target - nums[i].

Practice

1. What is the main purpose of using a hash map in the Two Sum Problem Classic Hash Solution?

easy

A. To store the sum of all pairs

B. To sort the array before searching

C. To reverse the array elements

D. To quickly check if the complement of the current number exists

Solution

1. Step 1: Understand the role of hash map

   The hash map stores numbers and their indices for quick lookup.

2. Step 2: Use hash map to find complement

   For each number, check if (target - number) exists in the map to find the pair.

3. Final Answer:

   To quickly check if the complement of the current number exists -> Option D

4. Quick Check:

   Hash map = fast complement check [OK]

Hint: Hash map helps find pairs in one pass [OK]

Common Mistakes:

• Thinking hash map sorts the array
• Trying to store sums instead of indices
• Ignoring the complement concept

2. Which of the following is the correct syntax to declare a dictionary in C# for the Two Sum problem?

easy

A. Dictionary map = new Dictionary();

B. var map = new List<int>();

C. HashMap map = new HashMap();

D. Dictionary map = new Dictionary();

Solution

1. Step 1: Identify correct dictionary declaration

   In C#, Dictionary<int, int> is used for key-value pairs.

2. Step 2: Check syntax correctness

   Dictionary map = new Dictionary(); correctly declares and initializes the dictionary.

3. Final Answer:

   Dictionary map = new Dictionary(); -> Option A

4. Quick Check:

   C# dictionary syntax = Dictionary map = new Dictionary(); [OK]

Hint: Use Dictionary for key-value pairs [OK]

Common Mistakes:

• Using List instead of Dictionary
• Using Java's HashMap syntax in C#
• Not specifying generic types

3. What will be the output of the following C# code snippet for input array [2, 7, 11, 15] and target 9?

public int[] TwoSum(int[] nums, int target) {
    Dictionary map = new Dictionary();
    for (int i = 0; i < nums.Length; i++) {
        int complement = target - nums[i];
        if (map.ContainsKey(complement)) {
            return new int[] { map[complement], i };
        }
        map[nums[i]] = i;
    }
    return new int[] { -1, -1 };
}

medium

A. [1, 2]

B. [0, 1]

C. [-1, -1]

D. [2, 3]

Solution

1. Step 1: Trace the loop and map updates

   At i=0, map is empty; add (2,0). At i=1, complement=9-7=2 found in map at index 0.

2. Step 2: Return indices of complement and current

   Return [0,1] as the pair sums to 9.

3. Final Answer:

   [0, 1] -> Option B

4. Quick Check:

   nums[0]+nums[1] = 2+7 = 9 [OK]

Hint: Check complement in map before adding current [OK]

Common Mistakes:

• Returning indices in wrong order
• Not updating map correctly
• Returning [-1, -1] incorrectly

4. Identify the bug in this Two Sum solution code snippet:

public int[] TwoSum(int[] nums, int target) {
    Dictionary map = new Dictionary();
    for (int i = 0; i <= nums.Length; i++) {
        int complement = target - nums[i];
        if (map.ContainsKey(complement)) {
            return new int[] { map[complement], i };
        }
        map[nums[i]] = i;
    }
    return new int[] { -1, -1 };
}

medium

A. Dictionary should be a List

B. Complement calculation is wrong

C. Loop condition should be < instead of <=

D. Return statement is missing

Solution

1. Step 1: Check loop boundary

   The loop uses i <= nums.Length which causes index out of range error.

2. Step 2: Correct loop condition

   Change to i < nums.Length to avoid accessing invalid index.

3. Final Answer:

   Loop condition should be < instead of <= -> Option C

4. Quick Check:

   Loop boundary off by one causes error [OK]

Hint: Use < length for array loops to avoid errors [OK]

Common Mistakes:

• Using <= instead of < in loop
• Confusing data structures
• Ignoring index out of range exceptions

5. Given the array [3, 3, 4, 5] and target 6, what will the Two Sum hash solution return and why?

hard

A. [0, 1] because the first two 3's sum to 6

B. [1, 2] because 3 and 4 sum to 6

C. [-1, -1] because no pair sums to 6

D. [2, 3] because 4 and 5 sum to 6

Solution

1. Step 1: Trace the map and complements

   At i=0, map empty, add (3,0). At i=1, complement=6-3=3 found at index 0.

2. Step 2: Return indices of matching pair

   Return [0,1] because nums[0]+nums[1]=3+3=6.

3. Final Answer:

   [0, 1] because the first two 3's sum to 6 -> Option A

4. Quick Check:

   Duplicate values handled correctly [OK]

Hint: Duplicates can form valid pairs if indices differ [OK]

Common Mistakes:

• Ignoring duplicates in map
• Assuming no pair exists with same number twice
• Returning wrong indices