The Runtime Theory
mediumLeetCode#sorting#two-pointers#deduplication

3Sum

Return unique triples whose values sum to zero; input indices do not matter.

The Runtime Theory Team25 min read
Solve it

Solving happens on the judge — come back and mark it done

Sample cases

innums=[-1,0,1,2,-1,-4]

out[[-1,-1,2],[-1,0,1]]

innums=[0,1,1]

out[]

innums=[0,0,0]

out[[0,0,0]]

Return unique triples whose values sum to zero; input indices do not matter.

A good solution should

  • State the invariant or decision rule that makes the approach correct.
  • Explain the time and space complexity in terms of the input sizes and required output.
  • Handle empty, minimal, duplicate, and boundary-shaped inputs covered by the contract.

Reasoning prompt

Sort first, then fix one value and sweep inward. Explain how you skip duplicates without skipping distinct valid triples.

Use the linked judge for the canonical problem. The examples above are compact TRT checks; write additional cases before submitting, especially for the boundary that tends to break your chosen invariant.

More practice in this topic

medium · two pointersContainer With Most Waterhard · two pointersTrapping Rain Watermedium · two pointersTwo Sum II - Input Array Is Sorted

One dispatch a week

The trace behind each problem, the tradeoff that explains it, and one technical dispatch per week — no noise.

One technical dispatch per week. No noise.

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