F - Warm up HDU - 축소점 dp 트리 지름

F - Warm up

#include
using namespace std;
#define LL long long
const int INF=0x3f3f3f3f;
const int N=200010;
const int M=1000010;
struct Edge
{
    int v,nt,flag;
} edge[M<<2];
int s[N],cnt;
int n,m;
int dfn[N],low[N],dep,vis[N];
int id[N],res;
bool instack[N];
int nbridge,bridge[M][2];
int maxlen;
stackst;
void AddEdge(int u,int v)
{
    edge[cnt].v=v;
    edge[cnt].nt=s[u];
    edge[cnt].flag=1;
    s[u]=cnt++;
    edge[cnt].v=u;
    edge[cnt].nt=s[v];
    edge[cnt].flag=1;
    s[v]=cnt++;
}
void tarjan(int u)
{
    dfn[u]=low[u]=++dep;
    st.push(u);
    instack[u]=1;
    for(int i=s[u]; i!=-1; i=edge[i].nt)
    {
        int v=edge[i].v;
        if(edge[i].flag==0)continue;
        edge[i].flag=edge[i^1].flag=0;
        if(!dfn[v])
        {
            tarjan(v);
            low[u]=min(low[u],low[v]);
            if(low[v]>dfn[u])
            {
                bridge[nbridge][0]=u;
                bridge[nbridge++][1]=v;
            }
        }
        else if(instack[v])low[u]=min(low[u],dfn[v]);
    }
    if(dfn[u]==low[u])
    {
        int temp;
        do
        {
            id[temp=st.top()]=res;
            st.pop();
            instack[temp]=0;
        }
        while(temp!=u);
        res++;
    }
}
int dfs(int u,int pre)
{
    int temp=0;
    for(int i=s[u]; i!=-1; i=edge[i].nt)
    {
        int v=edge[i].v;
        if(v==pre)continue;
        int d=dfs(v,u);
        maxlen=max(maxlen,temp+d);
        temp=max(temp,d);
    }
    return temp+1;
}
int main()
{
    while(~scanf("%d%d",&n,&m)&&(n+m))
    {
        memset(s,-1,sizeof s);
        cnt=0;
        for(int i=0; i