Day 13/365: Dutch National Flag -- DSA & System Design

365 Day DSA & System Design Challenge

Day 13 was about making a famous array puzzle feel structurally simple.

What I wanted clear today:

• this is really a partition problem, not generic sorting
• low, mid, and high each protect a different region
• swapping with the right side does not always let mid advance
• invariants are what make the one-pass solution trustworthy
• the same idea shows up again in pivot partition problems later

The biggest shift was realizing that the algorithm becomes much easier once the array is described as promises. Left side already solved. Right side already solved. Middle region still unresolved. Then each swap has a reason and each pointer move has a constraint.

That made Day 13 feel much more like reusable pointer reasoning than a narrow interview trick.

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Day 13/365 of the 365 Day DSA & System Design Challenge.