The Runtime Theory
mediumLeetCode#binary-search#boundary-search

Find First and Last Position of Element

Find the inclusive first and last indices of target in a sorted array, or [-1,-1].

The Runtime Theory Team25 min read
Solve it

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

Sample cases

innums=[5,7,7,8,8,10], target=8

out[3,4]

innums=[5,7,7,8,8,10], target=6

out[-1,-1]

innums=[], target=0

out[-1,-1]

Find the inclusive first and last indices of target in a sorted array, or [-1,-1].

A good solution should

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

Reasoning prompt

Run lower and upper boundary searches in logarithmic time. Test duplicates spanning the midpoint.

Use the linked judge for the canonical challenge. The examples above are compact TRT checks; create additional tests around the boundary most likely to break your invariant.

More practice in this topic

easy · binary searchBinary Searchmedium · 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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