The Runtime Theory
easyLeetCode#binary-search#invariants

Binary Search

Return the index of target in a sorted array, or -1 when it is absent.

The Runtime Theory Team25 min read
Solve it

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

Sample cases

innums=[-1,0,3,5,9,12], target=9

out4

innums=[-1,0,3,5,9,12], target=2

out-1

innums=[5], target=5

out0

Return the index of target in a sorted array, or -1 when it is absent.

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

Choose a closed or half-open interval and maintain it consistently. Explain why each iteration strictly reduces the candidate range.

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 · binary searchFind First and Last Position of Elementmedium · binary searchFind Minimum in Rotated Sorted Arraymedium · binary searchKoko Eating Bananas

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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