We Need More Bosses CodeForces - 1000E（缩点 建图 求桥 求直径）

#include <bits/stdc++.h>
#define mem(a, b) memset(a, b, sizeof(a))
using namespace std;
const int maxn = 1e6+10, INF = 0x7fffffff;
vector<int> G[maxn<<1];
int pre[maxn<<1], lowlink[maxn<<1], sccno[maxn<<1], dfs_clock, scc_cnt, d[maxn<<1], vis[maxn<<1];
stack<int> S;
int n, m, maxway, pos;
map<int, int> w[maxn<<1];
int head[maxn<<1], cnt, ans;
struct node
{
    int u, v, next;
}Node[maxn<<1];

void add_(int u, int v)
{
    Node[cnt].u = u;
    Node[cnt].v = v;
    Node[cnt].next = head[u];
    head[u] = cnt++;
}

void add(int u, int v)
{
    add_(u, v);
    add_(v, u);
}


void dfs(int u, int fa)
{
    pre[u] = lowlink[u] = ++dfs_clock;
    S.push(u);
    for(int i=0; i<G[u].size(); i++)
    {
        int v = G[u][i];
        if(v == fa) continue;
        if(!pre[v])
        {
            dfs(v, u);
            lowlink[u] = min(lowlink[u], lowlink[v]);
        }
        else if(!sccno[v])
            lowlink[u] = min(lowlink[u], pre[v]);
    }
    if(lowlink[u] == pre[u])
    {
        scc_cnt++;
        for(;;)
        {
            int x = S.top(); S.pop();
            sccno[x] = scc_cnt;
            if(x == u) break;
        }
    }
}

void init()
{
    dfs_clock = scc_cnt = cnt = 0;
    mem(sccno, 0);
    mem(pre, 0);
    mem(head, -1);    
}

void bfs(int u)
{
    mem(vis, 0);
    mem(d, 0);
    queue<int> Q;
    Q.push(u);
    vis[u] = 1;
    d[u] = 0;
    maxway = 0, pos = u, ans = 0;
    while(!Q.empty())
    {
        int u = Q.front(); Q.pop();
        for(int i=head[u]; i!=-1; i=Node[i].next)
        {
            int v = Node[i].v;
            if(!vis[v])
            {
                vis[v] = 1;
                d[v] = d[u] + 1;
                if(d[v] > maxway)
                    maxway = d[v], pos = v;
                Q.push(v);
            }
        }
    }
}

int main()
{
    init();
    cin>> n >> m;
    int u, v;
    for(int i=0; i<m; i++)
    {
        cin>> u >> v;
        G[u].push_back(v);
        G[v].push_back(u);
    }
    dfs(1, -1);
//    cout<< scc_cnt <<endl;
    for(int i=1; i<=n; i++)
    {
        for(int j=0; j<G[i].size(); j++)
        {
            int v = G[i][j];
            if(sccno[i] != sccno[v])
            {
            //    cout<< i << "  " << v <<endl;
            //    w[i][v] = w[v][i] = 1;
                if(!w[sccno[i]][sccno[v]])
                    w[sccno[i]][sccno[v]] = w[sccno[i]][sccno[v]] = 1, add(sccno[i], sccno[v]);
            }        
        }
    }
    bfs(1);
    bfs(pos);
    cout<< maxway <<endl;
    

    return 0;
}