PythonIntermediate~3h

Interview: Two Pointers & Sliding Window

Recognition first, mechanics second

Most material teaches the two techniques and leaves recognition to luck, which is why people memorise solutions and freeze on a variant. This starts at the decision instead: given an unlabelled problem, does a window apply at all, which of the three motions does it want, and what breaks when the values can be negative.

Start this project See the modules

Modules: 4
Drills: 7
Build steps: 4
Time: ~3h

Walk out able to

Take an unseen array problem, say in ten seconds whether a window applies and which shape it wants, and write the loop with its edges right first time.

What this covers

The three motions

Closing in, trailing, and a window between two edges.

Does it apply at all?

Two questions that settle it before any code is written.

What negatives break

Not the window — the shrink. And what replaces it.

Edges and empty input

The six sums, and the calls that answer instead of failing.

The pitch

What you practise, and what you leave with

A fixed window completed from a skeleton, a variable window written from a specification, and a problem that must be refused before it can be solved.

You will practise

Which of the three motions a problem wants
What negatives do, and what they do not
Reading a nearly-right shrink for its one fault
Edges, widths, and degenerate input

Afterwards you can

Decide in ten seconds whether a window applies
Name the precondition a loop is leaning on
Write either template with its edges right
Refuse a problem, and say precisely why

Modules

4 modules, 19 items

Lessons explain one idea. Drills repeat it until it sticks. Build steps make something that exists afterwards.

1
Three shapes, one idea
~39 min
Tell the three pointer motions apart, and walk any of them to its stopping point.
- LessonWhy recognition, not templates4 min
- LessonTell the three motions apart8 min
- LessonWhat each motion is leaning on6 min
- DrillClose in from both ends11 min
- DrillWalk the writer forward10 min
2
Does a window even apply?
~43 min
Take an unlabelled problem and say whether a window works, which kind, and why.
- LessonTest a problem for a window7 min
- LessonWho chooses the width?6 min
- LessonWhy one pass is enough5 min
- DrillDecide before you write13 min
- DrillFind the broken shrink12 min
3
Where the off-by-one lives
~42 min
State a window’s width and count from its indices, and know what nothing does to it.
- LessonTwo edges and a width5 min
- LessonDo the six sums without thinking6 min
- DrillCount the edges9 min
- DrillWalk the window by hand12 min
- DrillNothing, one, or too few10 min
4
Write them, then refuse one
~50 min
Write both templates against real input, then meet a problem that needs neither.
- Build stepCount the busy windows14 min
- Build stepShrink to the shortest run16 min
- Build stepRefuse it before you solve it8 min
- Build stepCount the runs that come out level12 min