Contest Checklist

Before submitting

• Samples pass — all of them, including any in the notes
• Edge cases: $n=0$, $n=1$, all equal, all negative, maximum values, disconnected
• long long wherever a product can exceed $2\cdot 10^9$
• Modulus applied after every addition and multiplication
• Subtraction under a modulus normalised: ((a-b) % M + M) % M
• Globals cleared between test cases
• Array sizes correct (n+1, not n)
• Indexing consistent (0-based or 1-based)
• Recursion depth under $\sim 2\cdot 10^5$
• Output format exact — precision, “impossible” sentinel, newlines
• Complexity checked against the constraints
• Memory estimated

Overflow checklist

• int * int overflows before assignment to long long — write 1LL * a * b
• Graph distances almost always need long long
• mid = (l + r) / 2 → l + (r - l) / 2
• i * i <= n → i <= n / i
• Products near $10^{18}$ → __int128
• Prefix sums of $10^5$ values up to $10^9$ reach $10^{14}$

Graph checklist

• Directed or undirected?
• Weighted or unweighted? Negative weights?
• Connected, or multiple components?
• Self-loops or parallel edges?
• Is it a tree, DAG, bipartite, or general?
• Do you need a path, a count, or just a distance?
• Is $n$ or $m$ the binding constraint?

DP checklist

• What exactly is the state? Is it complete? Is it minimal?
• Are the transitions acyclic (is the evaluation order valid)?
• Base cases: unreachable states initialised to $\pm \infty$, not 0?
• Is a modulus required?
• Max, min, count, or probability?
• Does order matter — are you over- or under-counting?
• Knapsack: weight loop downward for 0/1, upward for unbounded
• Coin change: coins outside for combinations, amount outside for permutations
• Can the state be compressed? Is memory within limits?

Binary search checklist

• The predicate is defined precisely
• It is monotone — verify against a brute force if unsure
• The bounds provably contain the answer
• First-true or last-true chosen deliberately
• Real-valued search uses a fixed iteration count, not an epsilon condition

Geometry checklist

• Integers wherever possible; squared distances for comparisons
• One epsilon, scaled to the coordinate magnitude
• Degenerate cases: collinear, duplicate points, $n\le 2$, zero-length segments
• Orientation convention fixed (CCW positive) and polygons normalised
• Cross products checked for overflow

Contest debug checklist

• Edge cases: n=0, n=1, all equal, all negative, disconnected, impossible
• 0-based vs 1-based indexing
• Global arrays cleared between tests
• Parent / visited arrays initialised
• Modulus after every operation
• Sorted before binary search or two pointers
• Recursion depth over $2\cdot 10^5$ → iterative

Interactive problems

• Flush after every query (endl, or cout << flush)
• Read the response before the next query
• Respect the query limit
• Handle the “you are wrong” response if the protocol has one
• Test against a locally written interactor

The template header

#include <bits/stdc++.h>
using namespace std;
using ll = long long;
 
int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int T = 1;
    // cin >> T;
    while (T--) solve();
}

Check whether the problem has multiple test cases before writing the loop — and if it does, make sure everything is reset inside solve().

Useful pragmas

#pragma GCC optimize("O2,unroll-loops")
#pragma GCC target("avx2,popcnt")

Legitimate on Codeforces and often a 2-4× speedup for tight loops and bitset work. Check the judge allows them.

Time management

Situation	Action
Stuck 15+ minutes with no idea	switch problems
Idea but messy implementation	look for a simpler formulation
Two wrong answers on the same problem	stress test before a third submit
30 minutes left	secure existing solutions, do not start something new
Everything solved	re-check the risky ones

See also: Common Pitfalls · CP Workflow · Debugging