Recall & Review
beginner
What is the main idea behind the Two Sum Problem Classic Hash Solution?
Use a hash table (dictionary) to store numbers and their indices while checking if the complement (target - current number) exists. This allows finding the pair in one pass efficiently.
Click to reveal answer
beginner
Why is a hash table useful in the Two Sum problem?
It provides fast lookup (average O(1) time) to check if the complement of the current number exists, avoiding nested loops and reducing time complexity from O(n²) to O(n).
Click to reveal answer
beginner
In the Two Sum problem, what does the 'complement' mean?
The complement is the number that, when added to the current number, equals the target sum. It is calculated as target - current number.
Click to reveal answer
intermediate
What is the time complexity of the classic hash solution for Two Sum?
O(n), where n is the number of elements in the array, because each element is processed once and hash lookups are O(1) on average.
Click to reveal answer
beginner
What happens if no two numbers add up to the target in the Two Sum hash solution?
The function returns an empty result or indication that no valid pair exists, after checking all elements without finding a complement.
Click to reveal answer
What data structure is primarily used in the classic Two Sum hash solution?
✗ Incorrect
The hash table allows quick lookup of complements to find pairs efficiently.
What is the complement of number 3 if the target sum is 9?
✗ Incorrect
Complement = target - current number = 9 - 3 = 6.
What is the time complexity of the classic Two Sum hash solution?
✗ Incorrect
Each element is processed once with O(1) average lookup, so total O(n).
If the input array is [2, 7, 11, 15] and target is 9, what pair indices does the hash solution return?
✗ Incorrect
2 + 7 = 9, indices are 0 and 1.
What happens if the complement is found in the hash table during iteration?
✗ Incorrect
Finding the complement means the pair is found, so return their indices immediately.
Explain how the classic hash solution for the Two Sum problem works step-by-step.
Think about how you find pairs quickly without checking every pair.
You got /6 concepts.
Describe why the hash table approach is more efficient than the brute force method for Two Sum.
Focus on how lookup speed changes the overall work.
You got /4 concepts.