Two Sum Problem Classic Hash Solution in DSA Python - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Two Sum Hash Map Solution

What is the output of the following Python code that solves the Two Sum problem using a hash map?

DSA Python

def two_sum(nums, target):
    lookup = {}
    for i, num in enumerate(nums):
        if target - num in lookup:
            return [lookup[target - num], i]
        lookup[num] = i

result = two_sum([2, 7, 11, 15], 9)
print(result)

A [0, 1]

B [1, 2]

C [2, 3]

D None

💡 Hint

Think about which two numbers add up to the target 9 and their indices.

The function finds two numbers that add up to 9. 2 (index 0) and 7 (index 1) sum to 9, so it returns [0, 1].

🧠 Conceptual

intermediate

Understanding Hash Map Usage in Two Sum

Why does the Two Sum hash map solution use a dictionary to store numbers and their indices?

A To quickly check if the complement (target - current number) exists in the list.

B To count how many times each number appears.

C To store all pairs of numbers that add up to the target.

D To sort the numbers before searching for pairs.

💡 Hint

Think about how to find the other number needed to reach the target efficiently.

The dictionary allows O(1) lookup to check if the complement exists, making the solution efficient.

❓ Predict Output

advanced

Output with No Valid Pair

What will be the output of this Two Sum hash map code when no two numbers add up to the target?

DSA Python

def two_sum(nums, target):
    lookup = {}
    for i, num in enumerate(nums):
        if target - num in lookup:
            return [lookup[target - num], i]
        lookup[num] = i
    return None

result = two_sum([1, 2, 3, 4], 8)
print(result)

A Error

B [0, 3]

C []

D None

💡 Hint

Check if any two numbers sum to 8 in the list.

No two numbers in [1, 2, 3, 4] add up to 8, so the function returns None.

🔧 Debug

advanced

Identify the Error in Two Sum Code

What error does this code produce when run, and why?

DSA Python

def two_sum(nums, target):
    lookup = {}
    for i, num in enumerate(nums):
        if target - num in lookup:
            return [lookup[target - num], i]
        lookup[num] = i

result = two_sum([3, 3], 6)
print(result)

A IndexError because of out of range index

B No error, output is [0, 1]

C TypeError due to wrong data type in lookup

D KeyError because lookup is accessed incorrectly

💡 Hint

Check how the function handles duplicates and if the lookup dictionary is accessed safely.

The code correctly finds indices [0, 1] because 3 + 3 = 6 and both indices are stored properly.

🚀 Application

expert

Number of Unique Pairs Summing to Target

Given the list [1, 2, 3, 4, 3, 2, 1] and target 4, how many unique pairs of indices (i, j) with i < j exist such that nums[i] + nums[j] == target?

A 3

B 4

C 5

D 6

💡 Hint

Consider pairs like (0,2), (1,5), and count unique index pairs carefully.

Pairs are (0,2), (0,4), (1,5), (2,6), (4,6). Total 5 unique pairs.