Diameter_Tree.cpp

#include<bits/stdc++.h>
using namespace std;
#define ll long long

#define ff        first
#define ss        second
#define sz(x)    (int) x.size()
#define endl      '\n'
#define pb        push_back
#define all(v)    v.begin(), v.end()
#define     YES   printf("YES\n")
#define     Yes   printf("Yes\n")
#define     NO    printf("NO\n")
#define     No    printf("No\n")

#define     mem(a)      memset(a , 0 ,sizeof a)
#define     memn(a)     memset(a , -1 ,sizeof a)

const int lim = 1048576;
const int M    = 1e9 + 7;
const int Inf           = (int)2e9 + 5;
const ll  Lnf           = (ll)2e18 + 5;
const int N             = 5e5 + 5;
const int NN            = 1e6 + 5;

#define error(args...) {vector<string>_v=split(#args,',');err(_v.begin(),args);cout<<endl;}
vector<string> split(const string &s, char c) {vector<string>v; stringstream ss(s); string x; while (getline(ss, x, c))v.emplace_back(x); return move(v);} void err(vector<string>::iterator it) {}
template<typename T, typename... Args>void err(vector<string>::iterator it, T a, Args...args) {cout << it->substr((*it)[0] == ' ', it->length()) << " = " << a << " "; err(++it, args...);}

std::vector<int>g[N];
int depth[N];
void dfs(int vertex, int par = 0)
{
	for (auto child : g[vertex])
	{
		if (child == par)continue;
		depth[child] = depth[vertex] + 1;
		dfs(child, vertex);
	}
}
int solve()
{
	int n, m;
	cin >> n >> m;
	for (int i = 0; i < m; ++i)
	{
		int x, y;
		cin >> x >> y;
		g[x].push_back(y);
		g[y].push_back(x);
	}
	dfs(1);
	int max_depth = -1;
	int max_depth_node;
	for (int i = 1; i <= n; ++i)
	{
		if (max_depth < depth[i])
		{
			max_depth = depth[i];
			max_depth_node = i;
		}
		depth[i] = 0;
	}

	dfs(max_depth_node);

	for (int i = 1; i <= n; ++i)
	{
		if (max_depth < depth[i])
		{
			max_depth = depth[i];
			max_depth_node = i;
		}
	}

  cout<<max_depth<<endl;

	return 0;
	//error();
}
int main() {

	//ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
	int test = 1, tc = 0;
	//cin >> test;
	while (test--) {
		//printf("Case %d: ", ++tc);
		solve();
	}

	return 0;
}