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easyDSA

How do you solve the Two Sum problem efficiently?

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01

Understand the problem

Explain Two Sum.

arrayshashing
02

Attempt it yourself

Sketch your approach before reading the solution — that's what interviews test.

Nudge consolestandby

Stuck? Beam a request up — the console returns a conceptual nudge that guides your logic without spoiling the implementation.

03

Study the solution

Use a hash map: for each number, check if target - num is already in the map; if so return the indices, else store num → index. This is O(n) time, beating the O(n²) brute-force double loop.

Solution ready — 2 min read

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04

Read the code

One-pass hash map
Run Playground
def two_sum(nums, target):
    seen = {}                       # value -> index
    for i, num in enumerate(nums):
        if target - num in seen:
            return [seen[target - num], i]
        seen[num] = i
    return []


# --- demo ---
print(two_sum([2, 7, 11, 15], 9))   # [0, 1]
05

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