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Two Sum coding interview

Solve Two Sum: return indices of two numbers that add to target. Explain time/space and edge cases.

Answers use simple, clear English.

Audio N/A

Time: O(n)Space: O(n)

Quick interview answer

One-pass hash map: for each value store index; look up target-x before inserting. O(n) time, O(n) space. Edges: duplicates, no solution, negative numbers, empty array.

Detailed answer

One-pass hash map: for each value store index; look up target-x before inserting. O(n) time, O(n) space. Edges: duplicates, no solution, negative numbers, empty array. Prefer indices not values when asked. Alternative: sort + two pointers loses original indices unless you track them.

Real example & use case

Promo engine: find two cart lines totaling gift-card amount.

Pros & cons

Pros: optimal average. Cons: extra memory vs sorted two-pointer.

Code example

def two_sum(nums: list[int], target: int) -> list[int]:
    seen: dict[int, int] = {}
    for i, x in enumerate(nums):
        need = target - x
        if need in seen:
            return [seen[need], i]
        seen[x] = i
    raise ValueError('no pair')

Practice code · python (view only · no execution)

def two_sum(nums: list[int], target: int) -> list[int]:
    seen: dict[int, int] = {}
    for i, x in enumerate(nums):
        need = target - x
        if need in seen:
            return [seen[need], i]
        seen[x] = i
    raise ValueError('no pair')

#dsa#hashmap#two-sum