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Finding Two Sum Indices Quiz

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Questions and Answers

What role does the dictionary play in solving the problem of finding two numbers that add up to a target?

It allows storing multiple pairs of numbers at once.

It enables O(1) lookup time for previously seen numbers. (correct)

It stores numbers as values and their indices as keys.

It aids in sorting the list before iterating through it.

What is the time complexity of the solution provided for finding the indices?

O(n log n)

O(log n)

O(n^2)

O(n) (correct)

In what scenario will the algorithm return an empty list?

When the list contains less than two numbers.

If no two numbers in the list add up to the target. (correct)

If two numbers that add up to the target are the same.

When all numbers in the list are the same.

What happens when a complement is found during the iteration?

The indices of the current number and its complement are returned. Signup and view all the answers

What is the space complexity associated with the algorithm?

O(n) due to storing every number and index in the dictionary. Signup and view all the answers

Why use the function `enumerate()` in Python?

It gives us both the index and the value of each element in a list. Signup and view all the answers

Study Notes

Finding Two Sum Indices

Use a dictionary (seen) to store numbers and their indices for fast lookups (O(1)).
Iterate through the input list (nums) using enumerate to get both the number and its index.
For each number (num), calculate its complement (secondNum = target - num).
Check if the complement exists as a key in the seen dictionary.
If the complement exists, return the current index (i) and the index of the complement from the seen dictionary.
If no complement is found, store the current number (num) and its index (i) in the seen dictionary.
If no pair is found after iterating through the entire list, return an empty list ([]).

Time and Space Complexity

Time Complexity: O(n) – The algorithm iterates through the input list only once.
Space Complexity: O(n) – In the worst case, all numbers and their indices are stored in the seen dictionary.

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Description

Test your understanding of the Two Sum problem using dictionaries for efficient lookups. This quiz covers the entire algorithm, including time and space complexity analysis. Perfect for students studying data structures and algorithms.