题意：给定无向图，问任意找$s$到$t$，路径上必须经过的边最多有几条
缩点之后找树的直径就好了qwq
```
#include<cstdio>
#include<cstring>
#include<cctype>
#include<queue>
#include<algorithm>
#define reg register
using namespace std;
const int N=3e5+5;
struct node
{
int to,nxt;
}edge[N<<1];
int n,m,num,head[N],dfn[N],low[N],stack[N];
int top,cnt,bel[N],tot,fr[N],to[N],dis[N],ans;
bool exist[N];
inline int read()
{
int x=0,w=1;
char c=getchar();
while (!isdigit(c)&&c!='-') c=getchar();
if (c=='-') c=getchar(),w=-1;
while (isdigit(c))
{
x=(x<<1)+(x<<3)+c-'0';
c=getchar();
}
return x*w;
}
inline void add_edge(int from,int to)
{
edge[++num]=(node){to,head[from]}; head[from]=num;
edge[++num]=(node){from,head[to]}; head[to]=num;
}
void tarjan(int k,int fa)
{
int temp;
dfn[k]=low[k]=++cnt;
stack[++top]=k; exist[k]=1;
for (reg int i=head[k];i;i=edge[i].nxt)
{
int v=edge[i].to;
if (v==fa) continue;
if (!dfn[v])
{
tarjan(v,k);
low[k]=min(low[k],low[v]);
}
else if (exist[v])
low[k]=min(low[k],dfn[v]);
}
if (dfn[k]==low[k])
{
++tot;
do
{
temp=stack[top--];
exist[temp]=0;
bel[temp]=tot;
}while (temp!=k);
}
}
void dfs(int k,int fa)
{
dis[k]=dis[fa]+1;
for (reg int i=head[k];i;i=edge[i].nxt)
{
int v=edge[i].to;
if (v!=fa) dfs(v,k);
}
}
int main()
{
n=read(),m=read();
for (reg int i=1;i<=m;i++)
{
fr[i]=read(),to[i]=read();
add_edge(fr[i],to[i]);
}
for (reg int i=1;i<=n;i++) if (!dfn[i]) tarjan(i,i);
memset(head,0,sizeof(head)); num=0;
for (reg int i=1;i<=m;i++)
if (bel[fr[i]]!=bel[to[i]]) add_edge(bel[fr[i]],bel[to[i]]);
dfs(1,0); int pos=1;
for (reg int i=2;i<=tot;i++) if (dis[i]>dis[pos]) pos=i;
dfs(pos,0);
for (reg int i=1;i<=tot;i++) ans=max(ans,dis[i]);
printf("%d\n",ans-1);
return 0;
}
```