The Runtime Theory
mediumLeetCode#three-way-partition#in-place

Sort Colors

Sort an array containing only 0, 1, and 2 in place without a library sort.

The Runtime Theory Team25 min read
Solve it

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

Sample cases

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

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

innums=[2,0,1]

out[0,1,2]

innums=[0]

out[0]

Sort an array containing only 0, 1, and 2 in place without a library sort.

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

Maintain low, scan, and high regions. Explain why the scan index should not advance after swapping an unknown value from the high end.

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 · arraysContains Duplicatemedium · arraysEncode and Decode a List of Stringsmedium · arraysGroup Anagrams

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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