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Bulgarian Informatics Group D · 2007–2026

270 independently explained problems: 54 Municipal, 54 Regional, 105 National Olympiad and 57 Autumn Tournament tasks. Start with the easier tasks and work upwards. Difficulty labels are learning estimates, not official ratings; Group D also includes challenging national-level problems. The six 2017 National tasks shared by Groups C and D use their existing permanent URLs. Years follow Arena’s competition-season labels. Source clarifications and special grader requirements are noted on affected tasks.

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Round / tournamentYearContent

Easy96

Counting 2

Digits 1

Arithmetic 1

Simulation 1

Sorting 2

Big integers 1

Counting 1

Number Theory 1

Strings 1

Arrays and counting 2

Simulation 1

Frequency counting 5

Grid simulation 1

Arithmetic indexing 1

Integer indexing 1

Sets and distinct values 1

Grid scanning 1

Sorting and checking 1

String conversion 1

Run counting 1

Arithmetic and greedy choice 1

Triangle inequalities 1

Combinatorial counting 1

Contribution counting 1

Geometry and boundary updates 1

Prefix sums 2

Arithmetic and digit scanning 1

String arithmetic 1

Digit sequences 1

Strings and positional weights 1

Character frequencies 1

String parsing 2

String scanning 2

Array scanning 2

Prime exponents 1

Permutations 1

Inverse permutations 1

Sequence scanning 1

Circular simulation 1

Digits and divisibility 1

Combinatorics 1

Number bases 1

Geometry enumeration 1

Digit frequency counting 1

Prefix sums and compact storage 1

Circular arrays 1

Periodic simulation 1

Digit arithmetic 1

Suffix maxima 1

Bounded enumeration 2

Sorting by elapsed time 1

Sequence simulation 2

Time simulation 1

Distinct graph neighbours 1

Coordinate geometry 1

Binary Search 1

Interval intersections 1

Calendar arithmetic 1

Geometric sums 1

Previous and next occurrence arrays 1

Precomputation 1

Sorting and simulation 1

Range queries 1

GCD and divisibility 1

Sequence generation 1

Grid rotation 1

Set intersection 1

LCM and arithmetic progressions 1

Euclidean algorithm 1

Window enumeration 1

Bijective base conversion 1

Frequency counting and sorting 1

Divisor enumeration 1

Two pointers on sorted arrays 1

Periodic prefix sums 1

Sorting distinct values 1

Numeral systems 1

Difference arrays 1

Digit sorting 1

Grid components 1

Sequence trend runs 1

Calendar simulation 1

Integer halving bounds 1

Medium142

Ranked Choice Simulation 1

Exhaustive search 2

Arrays and matrices 1

Number theory 1

Simulation 2

Sorting 1

Enumeration 1

Two Pointers 1

Number Theory 1

Strings and simulation 1

Big integers 1

Binary search 1

Binary Search 2

Strings 2

Arrays and counting 1

Sorting and greedy 1

Two pointers 1

Simulation 1

Rational Arithmetic 2

Combinatorial geometry 1

Invariants 1

Modular arithmetic 1

Arithmetic interval counting 1

Binary enumeration 1

Unbounded dynamic programming 1

Streaming digit sequence 1

Big integers 1

Digits and modular arithmetic 1

Fixed-event simulation 1

Frequency counting 2

Bounded-state simulation 1

Two-dimensional prefix sums 1

Prime factorization 3

Frequency inversion 1

Precomputation 1

Prefix and suffix maxima 1

Sorting and greedy 1

Bounding rectangles 1

String runs and simulation 1

Exact rational arithmetic 1

Permutation cycles 1

Partition dynamic programming 1

Sorting and runs 1

Greedy two pointers 1

Sorting and decimal subtraction 1

Boyer–Moore voting 1

Sieve and prefix sums 1

Functional graphs 1

Prime sieve and combinatorics 1

Packed presence bitmap 1

Counting formulas 1

Implicit sequence indexing 1

Constructive geometry 1

Big integer base conversion 1

Prime sieve and prefix sums 1

Geometry and indexing 1

Greedy interval traversal 1

Sorting by concatenation 1

Modular arithmetic and dates 1

Linear recurrence 1

Greedy representation 1

Two Pointers 2

Ordered set and streaming 1

Divisor enumeration 1

Cyclic shifts 1

Breadth-first search 1

Grid dynamic programming 2

Prime sieve 1

Area bounds and enumeration 1

Big integer arithmetic 1

Unbounded knapsack 1

Divisors and formulas 1

Local invariant updates 1

Divisor sieve and prefix sums 1

Sets and symmetry 1

Decimal arithmetic 1

Blocked-grid runs 1

Stacks and subset enumeration 1

Odd divisors 1

Prefix sums 2

Rectangle enumeration 1

Bitmask enumeration 1

Priority queue simulation 1

Big integer doubling 1

Prefix and suffix products 1

Greedy swaps 1

Date enumeration 1

Josephus formula 1

Hypothesis enumeration 1

Grid path invariants 1

Greedy 1

Big integer multiplication 1

Matrix exponentiation 1

String permutation 1

Sorting and two pointers 1

Digit arithmetic 1

Two pointers on positions 1

Stacks and dynamic programming 1

Dynamic programming 1

Sorting and sliding windows 1

Segmented multiple marking 1

Pair-sum preprocessing 1

Periodic indexing 1

Digit dynamic programming 1

Grid scanning 1

Positional digit counting 1

Integer binary search 1

Counting 1

Coprime directions and inclusion-exclusion 1

Multi-source BFS 1

Piecewise volume simulation 1

LCM and modular arithmetic 1

Decimal convolution 1

Number Theory 1

Divisor pairs and LCM 1

Greedy matching 1

Sorting and grouping 1

Graphs 1

Greedy stack simulation 1

Robot Without Reversing 12

Hard32

String algorithms 1

Prefix Sums 1

Number Theory 1

Bitmasks 1

Fenwick order statistics 1

Monotonic stack 1

Bitset dynamic programming 1

Digit constraints 1

Big integers and enumeration 1

Number Theory 1

Fibonacci enumeration 1

Geometry and arithmetic sums 1

Prime sieve 1

Exact rational arithmetic 1

Linear recurrence and counting 1

Greedy digit allocation 1

Flood fill and geometry 1

Suffix DP and lexicographic reconstruction 1

Simulation 1

Monotonic stacks and contributions 1

Greedy string repair 1

Exhaustive search and construction 1

Lexicographic unranking 1

Coordinate compression and BFS 1

Divisors and remainder pairing 1

Cached grid simulation 1

Dijkstra on a hexagonal grid 1

Digit DP and big integers 1

Grid Hamiltonian cycle 1

Divisors and packing 1

Geometry and sweep line 1

Minimum-cost bipartite matching 1

Try the problem yourself before reading the analysis. Solutions use C++17. Special grader headers and compiler requirements are explained on affected tasks.

GROUP D2007 · MediumSorting

Happy Years

Autumn Tournament (2007)

Submission name: happy.cpp

Solve in Editor →

Problem summary

A four-digit year is happy if exactly one pair of its digits is equal and the other two digits are different from that pair and from each other. Among the supplied unhappy years, find the one with minimum absolute difference from a supplied happy year. The winning unhappy year is unique. Print 0 if either kind is absent.

Input

N followed by N four-digit positive years.

Output

The uniquely closest unhappy year, or 0.

Constraints

0 < N < 9000; years are between 1000 and 9999.

Worked example

Original statement sample.

Input

4
1254 1987 1271 2002

Output

Solution analysis

Count the occurrences of the ten possible digits. The happy frequency pattern is 2,1,1: one digit occurs twice and no digit occurs three or four times. In particular, two pairs do not meet the word “exactly”.
Sort all supplied years, keeping their happy/unhappy labels. A closest pair with different labels can always be found at a boundary between two adjacent labels. Scan these boundaries and remember the unhappy endpoint of the smallest gap. Sorting costs far less than comparing every happy year with every unhappy year.
In the sample, 1271 is happy, whereas 2002 contains two pairs and is unhappy. Sorted order is 1254, 1271, 1987, 2002. The first boundary gap is 17, and the next is 716, so the answer is 1254.

Why it works

If a minimum-gap opposite-kind pair had an intervening year, that intervening year has one of the two kinds and forms a no-larger opposite-kind gap with an endpoint. Repeating gives an adjacent opposite-kind pair. The scan considers all such pairs and records their unhappy endpoints.

Complexity: O(N log N) time for sorting and O(N) storage for the years; classification uses constant space per year.

Watch out. Do not search among years absent from the input. Use absolute numerical distance, not digit distance. Exactly one repeated pair excludes two pairs and triples.

Commented C++17 solution

An independently written complete program with standard headers.

#include<algorithm>
#include<array>
#include<iostream>
#include<vector>
using namespace std;
bool happy(int year){
    array<int,10>frequency{};
    for(int digit=0;digit<4;digit++){
        frequency[year%10]++;
        year/=10;
    }
    int pairs=0;
    for(int count:frequency){
        if(count>2){
            return false;
        }
        if(count==2){
            pairs++;
        }
    }
    return pairs==1;
}
int main(){
    int n;
    cin>>n;
    vector<int>years(n);
    for(int&year:years){
        cin>>year;
    }
    sort(years.begin(),years.end());
    int bestGap=10000;
    int answer=0;
    for(int i=1;i<n;i++){
        bool leftHappy=happy(years[i-1]);
        bool rightHappy=happy(years[i]);
        // Only boundaries between the two kinds can improve the answer.
        if(leftHappy!=rightHappy&&years[i]-years[i-1]<bestGap){
            bestGap=years[i]-years[i-1];
            answer=leftHappy?years[i]:years[i-1];
        }
    }
    cout<<answer<<'\n';
}

Source: Arena Olimpiici ↗

Intermediate.Put your skills to the test.

Happy Years

Problem summary

Input

Output

Constraints

Worked example

Input

Output

Solution analysis

Why it works

Commented C++17 solution

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