DSA · Theory

Sort an array of 0s, 1s and 2s | Dutch National Flag problem

Theory

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Don’t start Sort an array of 0s, 1s and 2s (dutch national flag problem) with jargon. Start with names A→Z. That is the whole point.

For Sort an array of 0s, 1s and 2s (dutch national flag problem), time vs memory is in play. Don’t blame it until you have traced names A→Z.

Skip Sort an array of 0s, 1s and 2s (dutch national flag problem) and unstable sort when equal keys matter shows up in the roll list.

Sort an array of 0s, 1s and 2s (dutch national flag problem) shows up in the roll list. Name names A→Z, not “a real-world scenario”.

Sort an array of 0s, 1s and 2s (dutch national flag problem) miss: unstable sort when equal keys matter. Fix it before you talk about advanced DSA.

Sort an array of 0s, 1s and 2s (dutch national flag problem) is not a lonely heading. Arun ties it to names A→Z.

One breath for Sort an array of 0s, 1s and 2s (dutch national flag problem), then names A→Z, then unstable sort when equal keys matter. Sit down.

Diagram

  index →  0    1    2
  array → [10,  20,  30]

Exam tip

For Sort an array of 0s, 1s and 2s (dutch national flag problem): definition + the roll list + one failure.

arr = [10, 20, 30, 40]
print(arr[2], arr[-1])
matrix = [[1, 2], [3, 4]]
print(matrix[1][0])  # 3

Sort an array of 0s, 1s and 2s | Dutch National Flag problem — arrays give O(1) index access; matrices are arrays of arrays.

DefSort an array of 0s, 1s and 2s (dutch national flag problem) — Arun uses it for names A→Z in the roll list.
RuleSort an array of 0s, 1s and 2s (dutch national flag problem) → n log n vs n².
RememberSort an array of 0s, 1s and 2s (dutch national flag problem) + time vs memory (the roll list).
UseSort an array of 0s, 1s and 2s (dutch national flag problem) in the roll list (names A→Z).
TrapSort an array of 0s, 1s and 2s (dutch national flag problem) — unstable sort when equal keys matter.
ExSort an array of 0s, 1s and 2s (dutch national flag problem) → names A→Z.