Resuelve https://omegaup.com/arena/problem/votacion

votacion.cpp

#include <bits/stdc++.h>
using std::vector;
using std::pair;
using std::set;
using std::unordered_map;
using std::unordered_set;

void updateIndex(vector<int> const& ballot, int& index, unordered_set<unsigned> const& losers, int C) {
    while (index < C) {
        int candidate = ballot[index];
        if (losers.find(candidate) == end(losers)) {
            break;
        }
        index++;
    }
}

pair<unsigned, int> findMinResult(unordered_map<int, int> const& results) {
    pair<int, int> answer{0, std::numeric_limits<int>::max()};
    for (auto entry : results) {
        if (entry.second < answer.second) {
            answer = entry;
        }
    }
    return answer;
}

pair<unsigned, int> findMaxResult(unordered_map<int, int> const& results) {
    pair<int, int> answer{0, 0};
    for (auto entry : results) {
        if (entry.second > answer.second) {
            answer = entry;
        }
    }
    return answer;
}

unsigned findWinner(vector<vector<int>> const& ballots, int C, int V)  {
    vector<int> I(V, 0);
    unordered_set<unsigned> losers;
    for (int op = 0; op < C; op++) {
        unordered_map<int, int> results;
        for (int i = 0; i < V; i++) {
            int& index = I[i];
            auto ballot = ballots[i];
            updateIndex(ballot, index, losers, C);
            if (index >= C) {
                // We already processed the ballots C times and still have no winner. No one wins.
                return 0;
            }
            int candidate = ballot[index];
            results[candidate]++;
        }

        auto maxResult = findMaxResult(results);
        if (maxResult.second > 0.5L * V) {
            return maxResult.first + 1;
        }
        auto minResult = findMinResult(results);
        for (auto entry : results) {
            if (entry.second == minResult.second) {
                losers.insert(entry.first);
            }
        }
    }
    return 0; // No one wins
}

int main() {
    int C, V;
    std::cin >> C >> V;
    vector<vector<int>> ballots(V);
    for (int v = 0; v < V; v++) {
        for (int c = 0; c < C; c++) {
            int vote;
            std::cin >> vote;
            ballots[v].push_back(vote - 1);
        }
    }

    std::cout << findWinner(ballots, C, V) << "\n";
    return 0;
}