Longest Path in Tree/Diameter of a tree

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

#define fast ios_base::sync_with_stdio(0);cin.tie(NULL);cout.tie(NULL)
#define ll long long
#define ld long double
#define pb push_back
#define fe first
#define se second
#define nl "\n"
#define pp pair < ll , ll >
#define sz(x) (ll)x.size()
#define st(x) sort(x.begin(),x.end())
#define rst(x) sort(x.rbegin(), x.rend())
#define all(x) x.begin(),x.end()
long double pi = 3.14159265358979323;

const double EPS = 1e-12;
const int N = 1e6 + 5;
const int mod = 1e9 + 7;

bool vis[N];
vector < ll > v[N];
ll dp[N];
//Diameter of a tree
ll ans;
void dfs(ll s){

    vis[s] = 1;
    for(auto it : v[s]){
        if(!vis[it]){
            dfs(it);
        }
    }

    multiset < ll  > st;
    st.insert(0);
    for(auto it : v[s])st.insert(dp[it]);
    auto itr = st.rbegin();
    dp[s] = 1 + *itr;
    //cout << dp[s] << " ";
    if(st.size() > 1){
        itr++;
        ans = max(ans,  dp[s] + *itr);
    }
    ans = max(ans , dp[s]);
        //cout << s << " " << dp[s] << "\n";
}


int main()
{
    fast;
    ll n, m;
    cin >> n >> m ;
    for(int i = 0 ; i < m ;i++ ){
          ll a , b;
          cin >> a >> b ;
          v[a].pb(b);
          v[b].pb(a);
    }
    dfs(1);
    cout << ans - 1;
























    return 0;

}