Sliding window
Find a Clean Signal
01. The question
A receiver records lowercase letter codes. Return the longest continuous segment with no repeated code, not just its length. If segments tie, return the earliest one.
Before revealing the idea, predict the result and describe what information you would need to track.
02. Worked example
Inputsignal = "cabinabc"
Output"cabin"
- The first five letters form "cabin", with no repeated code.
- The next a repeats the earlier a. Move the left edge past its previous position.
- The final window "inabc" also has five letters. Keep "cabin", because it appeared first.
Try your code
Define solve(data, parameter="") and return your result. Your code runs locally, not on a server.
Edit your test cases
Use 1-10 objects with data and an optional string parameter. Expected outputs are computed from the lesson. An optional expected field must be correct.
Input contract & reference code
Example data: "cabinabc". Parameter: "". Numeric parameters arrive as strings; convert them with int(parameter) when needed.
The example and generated cases use the question’s exact variant. This checks behavior on a small test batch—not performance on every possible input.
Python downloads only when you run code, using compressed runtime assets. Code is not saved or uploaded. Enable JavaScript to run it.
Printed output
Five-second execution limit per batch. Standard-library Python only; additional packages and network access are unavailable. Stop ends execution without blocking the lesson.
03. The idea
Try sketching your own approach first. What would you track, and why would it work?
Reveal the idea
- Remember where each letter last appeared.
- Move the left edge forward when a repeat would break the window.
- Save a new best start only when the valid window is strictly longer. Return the saved slice.
04. Watch it happen
Use Step to move slowly, or Play to follow the whole trace. Change a preset or load your own input.
Input & settings
Red = current item / blue = active window or cells / green = completed work. A value docks at its actual destination; the original input stays unchanged. Positions start at 0.
Working state / what changes
Ready to begin
Enable JavaScript to step through the animation. The worked example and solution below remain readable without it.
Python / why it changes
The line behind the step
def clean_signal(signal: str) -> str:
last_seen = {}
left = best_start = best_length = 0
for right, character in enumerate(signal):
if character in last_seen:
left = max(left, last_seen[character] + 1)
last_seen[character] = right
length = right - left + 1
if length > best_length:
best_start, best_length = left, length
return signal[best_start:best_start + best_length]How to use this walkthrough
Hover or focus a control for its label. Play forms the input and follows values into the working state. Step shows one change; Back and the slider restore exact snapshots. Reduced motion uses instant updates. Saved snapshots and sorted heap displays are teaching overhead, not algorithm complexity.
05. Python solution
def clean_signal(signal: str) -> str:
last_seen = {}
left = best_start = best_length = 0
for right, character in enumerate(signal):
if character in last_seen:
left = max(left, last_seen[character] + 1)
last_seen[character] = right
length = right - left + 1
if length > best_length:
best_start, best_length = left, length
return signal[best_start:best_start + best_length]06. Cost & edge cases
- Time
- O(n) expected
- Space
- O(min(n, alphabet size))
- An empty signal returns "". Input codes are lowercase a-z.
- "zzzz" returns "z".
- "noon" returns "no": never move left backward and keep the earliest tied answer.