Recall & Review
beginner
What is the main idea behind the Two Sum problem classic hash solution?
Use a hash map (dictionary) to store numbers and their indices while checking if the complement (target - current number) exists in the map to find the pair quickly.
Click to reveal answer
beginner
Why do we use a hash map in the Two Sum problem?
A hash map allows us to check if the complement number exists in constant time, making the solution efficient with O(n) time complexity.
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 input list, because each element is processed once and hash map lookups are O(1).
Click to reveal answer
beginner
In the Two Sum hash solution, what does the complement represent?
The complement is the number needed to add to the current number to reach the target sum (complement = target - current number).
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 list indicating that no valid pair exists after checking all elements.
Click to reveal answer
What data structure is primarily used in the classic Two Sum hash solution?
✗ Incorrect
The hash map stores numbers and their indices for quick lookup of complements.
What is the time complexity of the Two Sum classic hash solution?
✗ Incorrect
Each element is processed once, and hash map lookups are O(1), so total is O(n).
In the Two Sum problem, what does the complement equal?
✗ Incorrect
Complement is the number needed to reach the target sum with the current number.
If the input list is [2, 7, 11, 15] and target is 9, what pair indices does the Two Sum solution return?
✗ Incorrect
2 + 7 = 9, so indices 0 and 1 are returned.
What does the Two Sum hash solution return if no pair sums to the target?
✗ Incorrect
It returns an empty list indicating no valid pair found.
Explain how the classic hash solution for the Two Sum problem works step-by-step.
Think about storing numbers and checking complements while looping.
You got /7 concepts.
Why is the hash map approach better than a nested loop for the Two Sum problem?
Compare time complexity and lookup speed.
You got /4 concepts.