Codeforces-solutions/Educational Codeforces Round 46 (Rated for Div. 2)/E. We Need More Bosses.cpp

/* Problem URL: https://codeforces.com/contest/1000/problem/E */

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

using namespace std;
using namespace __gnu_pbds;

template <class T, class comp = less<>>
using ordered_set = tree<T, null_type , comp , rb_tree_tag , tree_order_statistics_node_update>;

#define V vector

#define rmin(a, b) a = min(a, b)
#define rmax(a, b) a = max(a, b)

#define rep(i, lim) for (int i = 0; i < (lim); i++)
#define nrep(i, s, lim) for (int i = s; i < (lim); i++)

#define repv(i, v) for (auto &i : (v))
#define fillv(v) for (auto &itr_ : (v)) { cin >> itr_; }
#define sortv(v) sort(v.begin(), v.end())
#define all(v) (v).begin(), (v).end()

using vi = vector<int>;
using vvi = vector<vi>;
using vvvi = vector<vvi>;
using vvvvi = vector<vvvi>;

using ll = long long;

using vl = vector<ll>;
using vvl = vector<vl>;
using vvvl = vector<vvl>;
using vvvvl = vector<vvvl>;

template<class v>
auto operator<<(ostream &os, const vector<v> &vec)->ostream& {
    os << vec[0];
    for (size_t i = 1; i < vec.size(); i++) {
        os << ' ' << vec[i];
    }
    os << '\n';
    return os;
}

template<class v>
auto operator>>(istream &is, vector<v> &vec)->istream& {
    for (auto &i : vec) {
        is >> i;
    }
    return is;
}

template<class v>
auto operator<<(ostream &os, const vector<vector<v>> &vec)->ostream& {
    for (auto &i : vec) {
        os << i[0];
        for (size_t j = 1; j < i.size(); j++) {
            os << ' ' << i[j];
        }
        os << '\n';
    }
    return os;
}

template<class v>
auto operator>>(istream  &is, vector<vector<v>> &vec)->istream& {
    for (auto &i : vec) {
        for (auto &j : i) {
            is >> j;
        }
    }
    return is;
}

set<pair<int, int>> inv;
V<pair<int, int>> act;
int c = 0;

void IS_BRIDGE(int v,int to) {
    inv.emplace(v, to);
    inv.emplace(to, v);
    act.emplace_back(v, to);
    c++;
}; // some function to process the found bridge
int n; // number of nodes
vector<vector<int>> adj; // adjacency list of graph

vector<bool> visited;
vector<int> tin, low;
int timer;

void dfs(int v, int p = -1) {
    visited[v] = true;
    tin[v] = low[v] = timer++;
    bool parent_skipped = false;
    for (int to : adj[v]) {
        if (to == p && !parent_skipped) {
            parent_skipped = true;
            continue;
        }
        if (visited[to]) {
            low[v] = min(low[v], tin[to]);
        } else {
            dfs(to, v);
            low[v] = min(low[v], low[to]);
            if (low[to] > tin[v])
                IS_BRIDGE(v, to);
        }
    }
}

void find_bridges() {
    timer = 0;
    visited.assign(n, false);
    tin.assign(n, -1);
    low.assign(n, -1);
    for (int i = 0; i < n; ++i) {
        if (!visited[i])
            dfs(i);
    }
}

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int m;
    cin >> n >> m;
    adj.resize(n);

    while (m--) {
        int a, b;
        cin >> a >> b;
        a--, b--;
        adj[a].push_back(b);
        adj[b].push_back(a);
    }

    find_bridges();

    int g = 0;

    V<bool> vis(n);

    vi group(n);

    function<void(int)> att = [&](int i) {
        vis[i] = true;
        group[i] = g;

        for (auto j : adj[i]) {
            if (vis[j] || inv.find({i, j}) != inv.end()) {
                continue;
            }

            att(j);
        }
    };

    rep(i, n) {
        if (vis[i]) {
            continue;
        }
        att(i);
        g++;
    }

    vvi graph(c + 1);

    for (auto [u, v] : act) {
        graph[group[u]].push_back(group[v]);
        graph[group[v]].push_back(group[u]);
    }

    int oo = INT32_MAX >> 1;

    auto bfs = [&](int u) -> pair<int, int> {
        vi dis(c + 1, oo);
        dis[u] = 0;
        queue<int> q;
        q.push(u);

        int ans = u;

        while (!q.empty()) {
            auto i = q.front();
            q.pop();

            for (auto j : graph[i]) {
                if (dis[j] > dis[i] + 1) {
                    dis[j] = dis[i] + 1;
                    q.push(j);
                    ans = j;
                }
            }
        }

        return {ans, dis[ans]};
    };

    auto [u, _] = bfs(0);

    auto [v, ans] = bfs(u);

    cout << ans << '\n';
}