9.find Diameter of a Tree

 #include <bits/stdc++.h>

#include <ext/pb_ds/assoc_container.hpp>

#include <ext/pb_ds/tree_policy.hpp>

using namespace std;

using namespace __gnu_pbds;

template <class type1>

#define ll long long int

#define endl "\n"

#define ordered_set tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update>

using ordered_multiset = tree <type1, null_type, less_equal <type1>, rb_tree_tag, tree_order_statistics_node_update>;

 //ordered_multiset <ll> kek;(declaration for multiorder set)

// ordered_set o_set;(declaration)

//kek.order_of_key(i);(strictly less then i for multi order set)

//o_set.order_of_key(5) ;(strictly less then i for multi order set)

#define yes cout << "YES\n"

#define no cout << "NO\n"

#define N 100000

ll dept[N];

vector<ll>adj[N];

void dfs(ll node,ll par)

{

     for(auto u:adj[node])

     {

         if(u==par)continue;

         dept[u]=dept[node]+1;

         dfs(u,node);

     }

}

int main()

{

    ios_base::sync_with_stdio(0);

    cin.tie(0);

    cout.tie(0);

   

  ll n,m,i,j;

  cin>>n;

  for(i=1;i<=n-1;i++)

  {

      ll u,v;

      cin>>u>>v;

      adj[u].push_back(v);

      adj[v].push_back(u);

  }

  dfs(1,-1);

  ll dep=-1;

  ll node;

  for(i=1;i<=n;i++)

  {

     if(dept[i]>dep)

     {

         dep=dept[i];

         node=i;

     }

     dept[i]=0;

  }

  dfs(node,-1);

  dep=-1;

    for(i=1;i<=n;i++)

  {

     if(dept[i]>dep)

     {

         dep=dept[i];

       

     }

   

  }

  cout<<dep<<endl;

    return 0;

}