The Runtime Theory
mediumLeetCode#sliding-window#last-seen-index

Longest Substring Without Repeating Characters

Find the maximum length of a contiguous substring containing no repeated characters.

The Runtime Theory Team25 min read
Solve it

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

Sample cases

ins="abcabcbb"

out3

ins="bbbbb"

out1

ins="pwwkew"

out3

Find the maximum length of a contiguous substring containing no repeated characters.

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

Keep the left boundary monotonic. When a repeated character lies before the current window, do not move the boundary backward.

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 Replacementhard · sliding windowMinimum Window Substring

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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