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Codeforces Short Round 01

This pack is for Codeforces-style short-round shipping: one fast banker, one compact constructive slot, one hack-sensitive feasibility slot, and one optional late model.

Who This Is For

Use this pack if:

  • you already know the core families here in isolation
  • your main Codeforces losses come from slow shipping or weak anti-hack habits
  • you want a short, repeatable block that feels closer to a live CF round than a long archive grind

Entry Gate

Try this pack only if these already feel like review:

At minimum, these should no longer feel like first exposure:

Pack Shape

  • Type: Codeforces short-round shipping drill
  • Topic mix: constructive + feasibility + adversarial checking + late-slot graph modeling
  • Problems: 4
  • Suggested time: 2-2.5 hours

Topics Under Test

  • banker-first ordering in a short round
  • fast construction without overexplaining the entire proof to yourself
  • anti-hack discipline on feasibility-heavy code
  • knowing when to park an optional late slot instead of burning the whole round on it

Suggested Order

Slot Problem Topic Why it is here
1 Same Parity Summands constructive banker Fast first ship: small statement, quick parity split, and low setup cost.
2 Build the Permutation constructive / witness building Typical CF mid-early slot: easy to overcomplicate if you do not lock one clean construction fast.
3 Prefix Sum Addicts feasibility / anti-hack Strong slot for practicing boundary checks, impossible cases, and “looks right on samples” traps.
4 The Door Problem 2-SAT Optional late slot: compact graph transform, but only worth opening after the earlier ships are stable.

How To Run It

  1. spend 8-10 minutes reading all 4 slots
  2. classify each as:
  3. ship now
  4. one idea away
  5. park
  6. solve in the suggested order unless you can justify swapping 1 and 2
  7. after each supposedly-finished slot, spend 3-5 minutes on one anti-hack pass:
  8. smallest legal case
  9. impossible or parity boundary
  10. duplicate / degenerate family
  11. do not open slot 4 if slot 3 still has a vague correctness story

This pack is about shipping and hardening, not about starting the fanciest slot earliest.

Allowed Refreshers

Debrief

After the pack, log:

  1. first slot that should have shipped earlier
  2. first bug or fear that was really an anti-hack miss, not a theorem miss
  3. whether slot 4 was opened too early, too late, or at the right time
  4. one concrete next action:
  5. topic revisit
  6. implementation clinic
  7. constructive / validator-first clinic
  8. upsolve worksheet

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