Tree Diameter.cpp

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
using namespace std;
#define AB_BHI_NI_DEGI                \
    ios_base::sync_with_stdio(false); \
    cin.tie(NULL);                    \
    cout.tie(NULL);
#define int long long
#define pb push_back
#define N 100009
#define inf 1e18
const double PI = 3.141592653589793238462643383279;
int mod = 1e9 + 7;
//int mod = 998244353;
#define P pair<int, int>
#define F first
#define S second
#define all(v) v.begin(), v.end()
#define vi vector<int>
#define ld long double
#define ordered_set tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update>
 
vector<int> G[2 * N];
int dis[2 * N];
void bfs(int src, int n)
{
    for (int i = 1; i <= n; i++)
        dis[i] = 0;
    int vis[n + 1] = {0};
    vis[src] = 1;
    queue<int> q;
    q.push(src);
    while (q.size())
    {
        int sz = q.size();
        while (sz--)
        {
            int u = q.front();
            q.pop();
            for (int v : G[u])
            {
                if (!vis[v])
                {
                    vis[v] = 1;
                    dis[v] = dis[u] + 1;
                    q.push(v);
                }
            }
        }
    }
}
void solve()
{
    int n;
    cin >> n;
    for (int i = 2; i <= n; i++)
    {
        int u, v;
        cin >> u >> v;
        G[u].pb(v);
        G[v].pb(u);
    }
    bfs(1, n);
    int mx1 = 1;
    for (int i = 1; i <= n; i++)
    {
        if (dis[i] > dis[mx1])
            mx1 = i;
    }
    bfs(mx1, n);
    int ans = 0;
    for (int i = 1; i <= n; i++)
        ans = max(ans, dis[i]);
    cout << ans << "\n";
}
int32_t main()
{
    AB_BHI_NI_DEGI
    int Test = 1;
    //cin >> Test;
    //init();
    int itr = 1;
    while (Test--)
    {
        //cout << "Case #" << itr++ << ": ";
        solve();
    }
    return 0;
}