rt，把求树的直径的代码改一下就过了。整个过程不超过10min.

#include<bits/stdc++.h>
#define pb push_back
using namespace std;
int o[1000005],dep[1000005];
vector<int>g[1000005];
void dfs(int now,int fa){
	if(!fa) dep[now] = dep[fa] + g[now].size() + 1;//我儿子+我自己
	else dep[now] = dep[fa] + g[now].size() - 1;//我儿子-我爸爸
	for(auto x : g[now]) if(x != fa) dfs(x,now);
}
signed main(){
	int n,m;
	cin >> n >> m;
	for(int i = 1;i <= m;++i) {
		int u,v;
		cin >> u >> v;
		g[u].pb(v);
		g[v].pb(u); 
	}
	dfs(1,0);
	int nxt = 0;
	for(int i = 1;i <= n;++i) if(dep[i] > dep[nxt]) nxt = i;
	dfs(nxt,0);
	for(int i = 1;i <= n;++i) if(dep[i] > dep[nxt]) nxt = i;
	cout << dep[nxt] << '\n';
	return 0;
}