The Runtime Theory
mediumLeetCode#sliding-window#frequency-count

Permutation in String

Determine whether any contiguous window in the second string is a permutation of the first string.

The Runtime Theory Team25 min read
Solve it

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

Sample cases

ins1="ab", s2="eidbaooo"

outtrue

ins1="ab", s2="eidboaoo"

outfalse

ins1="", s2="abc"

outtrue

Determine whether any contiguous window in the second string is a permutation of the first string.

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 a fixed-size window equal to the first string length. Compare frequency counts without recounting the whole window at every shift.

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

easy · sliding windowBest Time to Buy and Sell Stockmedium · sliding windowLongest Repeating Character Replacementmedium · sliding windowLongest Substring Without Repeating Characters

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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