core practice · coding problem

Longest substring without repeats

Find the length of the longest substring with no repeating characters using a variable-size sliding window.

The mechanism

A sliding window maintains a contiguous range and an aggregate (sum, count, max) as it moves. Expand the right edge to include elements; shrink the left edge when a constraint breaks. Each element enters and leaves the window at most once — O(n).

Study Sliding Window

Expected evidence

lengthOfLongestSubstring('abcabcbb') -> 3

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Hints when you need them

Expand the right edge; on a duplicate, shrink the left edge until the window is valid again.
Track the last index of each character.

Watch for these mistakes

Recomputing the aggregate from scratch instead of updating incrementally
Shrinking with an `if` when a `while` is needed to restore the invariant
Confusing fixed-size and variable-size window templates

Check the sources

Practice the same mechanism

Longest Substring Without Repeating Characters

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