Mock Contest 1 -- Div 2 Warm-Up

Time limit: 2 hoursTarget audience: Codeforces 1100-1500

Simulate a real Codeforces Div 2 contest. Set a timer. No looking at editorials until time is up.

Quick Revisit

• Five-problem Div 2 warm-up (rated ~800 to ~1700) for time management and problem-ordering practice.
• Hints are intentionally hidden behind collapsible blocks below the problem table — for the simulation to be useful, do not expand them until you are stuck.
• Prerequisite: CF Tips and Workflow.

Problems

Open all five problems before starting the timer, then attempt them in your preferred order.

#	Problem	Expected Difficulty	Time Target
A	CF 1352A -- Sum of Round Numbers	800	5 min
B	CF 1352C -- K-th Not Divisible	1200	10 min
C	CF 1354B -- Ternary String	1200	15 min
D	CF 1360F -- Spy-Spy Interaction	1500	25 min
E	CF 1374E2 -- Reading Books (Hard)	1700	35 min

Hints (open only when stuck)

Problem A hint

Implementation — decompose into powers of 10.

Problem B hint

Math — count multiples in blocks.

Problem C hint

Sliding window — shortest window with all 3 characters.

Problem D hint

Constructive — checkerboard parity argument.

Problem E hint

Greedy + sorting by category.

Diagnostic Read-out

A single mock contest cannot reliably estimate your CF rating — it is too small a sample, and your luck on this particular problem set matters. Treat the table below as a diagnostic read-out: which gap does this run reveal, and what should you practice next?

Problems Solved	Diagnostic	Where to focus next
A only	Implementation speed and statement-reading still costing time	Tier 1 Speed Drills
A + B	Solid basics; B-level math comfort still uneven	More 1100-1300 problems on math + sliding windows
A + B + C	Comfortable in Div 2 A-C territory	Move on to D-level techniques (Mock 2, simple greedy + DP)
A + B + C + D	Strong Div 2 performer	E-level practice; harder DP and constructive problems
All 5	Confident across the band represented here	Try Mock 2 (technique diagnostic) and Educational rounds

Post-Contest Reflection

After the timer ends, answer these questions in your Mistake Journal:

1. Time management: How long did you actually spend on each problem? Where did you waste time?
2. Wrong approaches: Did you go down a wrong path? At what point should you have pivoted?
3. Implementation bugs: What bugs did you have? Could you have prevented them with better coding habits?
4. What you didn't know: Were there techniques you hadn't seen before? Add them to your study list.
5. Speed vs accuracy: Did you rush and make mistakes, or think too long and run out of time?

Detailed Walkthrough (read after attempting)

Problem A walkthrough

Decompose n into digits x their place value. E.g., 7231 = 7000 + 200 + 30 + 1. Loop through digits, output non-zero ones x 10^position. Common mistake: forgetting to skip zeros.

Problem B walkthrough

Every block of (n-1) consecutive non-multiples is followed by one multiple of n. k-th non-multiple = k + floor((k-1)/(n-1)). Common mistake: off-by-one in the formula.

Problem C walkthrough

Sliding window: maintain counts of {1,2,3}. Expand right until all present, then shrink left. Track minimum window size. Common mistake: not handling the shrink phase correctly.

Problem D walkthrough

Key insight: in any 2x2 square, opposite corners have equal values (parity argument). Assign values based on (i+j) % 2 coloring. Common mistake: not seeing the checkerboard pattern.

Problem E walkthrough

Categorize books: Alice-only, Bob-only, both, neither. Greedily pick "both" books first. Fill remaining slots with best single-person books, maintaining balance. Common mistake: not handling the case where you need equal counts from Alice-only and Bob-only.

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See also: Mock Contest 2 | Mock Contest 3 (Speed) | Mistake Journal | Practice Ladder 1100-1400