Recall & Review
beginner
What is the Four Sum problem?
The Four Sum problem asks to find all unique groups of four numbers in an array that add up to a given target sum.
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beginner
Why do we sort the array before solving the Four Sum problem?
Sorting helps to efficiently skip duplicates and use the two-pointer technique to find pairs that sum to a target.
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intermediate
What technique is commonly used to find quadruplets in the Four Sum problem?
We use nested loops for the first two numbers and then two pointers to find the remaining two numbers that complete the quadruplet.
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intermediate
How do you avoid duplicate quadruplets in the Four Sum problem?
Skip over numbers that are the same as the previous number at each step of the loops and pointers to ensure unique quadruplets.
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intermediate
What is the time complexity of the Four Sum solution using sorting and two pointers?
The time complexity is O(n^3) because of two nested loops and a two-pointer scan inside them.
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What is the first step before applying the two-pointer technique in Four Sum?
✗ Incorrect
Sorting the array helps to efficiently find pairs and skip duplicates.
How many nested loops are used before applying two pointers in Four Sum?
✗ Incorrect
Two nested loops fix the first two numbers, then two pointers find the other two.
What is the main reason to skip duplicate numbers in Four Sum?
✗ Incorrect
Skipping duplicates ensures only unique quadruplets are included.
Which data structure is primarily used to store the final quadruplets?
✗ Incorrect
Quadruplets are stored in a list or array for output.
What is the overall time complexity of the Four Sum solution?
✗ Incorrect
The solution uses two nested loops and two pointers, resulting in O(n^3) time.
Explain step-by-step how to find all unique quadruplets that sum to a target in an array.
Think about fixing two numbers and searching for pairs with two pointers.
You got /5 concepts.
Describe how skipping duplicates works in the Four Sum problem and why it is important.
Duplicates appear when consecutive numbers are equal after sorting.
You got /4 concepts.