The Runtime Theory
mediumLeetCode#two-pointers#sorted-input

Two Sum II - Input Array Is Sorted

Return one-based indices of two values in a nondecreasing array that sum to the target.

The Runtime Theory Team25 min read
Solve it

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

Sample cases

innumbers=[2,7,11,15], target=9

out[1,2]

innumbers=[2,3,4], target=6

out[1,3]

innumbers=[-1,0], target=-1

out[1,2]

Return one-based indices of two values in a nondecreasing array that sum to the target.

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

Use the sorted order to move one boundary at a time. State the invariant that rules out the discarded region.

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

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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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