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mediumDSA

How do you solve the 3Sum problem?

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01

Understand the problem

Explain finding triplets summing to zero.

arraystwo-pointers
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

Sort the array, then fix one element and use two pointers on the rest to find pairs summing to its negation. Skip duplicates at each level to avoid repeated triplets. Sorting is O(n log n) and the scan is O(n²) overall.

Solution ready — 2 min read

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04

Read the code

Sort + two pointers
Run Playground
def three_sum(nums):
    nums.sort()
    res = []
    for i in range(len(nums) - 2):
        if i > 0 and nums[i] == nums[i - 1]:
            continue                      # skip duplicate fixed value
        left, right = i + 1, len(nums) - 1
        while left < right:
            total = nums[i] + nums[left] + nums[right]
            if total < 0:
                left += 1
            elif total > 0:
                right -= 1
            else:
                res.append([nums[i], nums[left], nums[right]])
                left += 1; right -= 1
                while left < right and nums[left] == nums[left - 1]:
                    left += 1
                while left < right and nums[right] == nums[right + 1]:
                    right -= 1
    return res


# --- demo ---
print(three_sum([-1, 0, 1, 2, -1, -4]))   # [[-1, -1, 2], [-1, 0, 1]]
05

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