Debrief

What the last problem was actually testing, and what to do about whichever ones went badly.

Quick check· 5 min

Why map zero to negative one?

In the balanced-subarray solution, zeros are counted as -1 and ones as +1. What does that transformation buy?

It turns 'equal counts of two values' into 'running total returns to a previous value', which a prefix sum plus a map can answer in one passIt keeps the running total small, which avoids overflow on long inputsIt makes the array sorted, so two pointers can converge on the answerIt lets the window shrink from the left whenever the total goes negative

Select all that apply· 5 min

Match the tool to the question

Select every question below that a prefix array answers better than a sliding window.

Many queries asking for the sum between two given indexesLongest stretch summing to a target, where values may be negativeLongest stretch of positive values summing to at most a limitLongest stretch with equally many zeros and onesLongest stretch with no repeated character

Checkpoint: arrays, pointers, and windows

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Checkpoint: arrays, pointers, and windowsLesson 3 of 3About 17 min