Finding cetnroid of a tree.cpp

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


//  ->uncomment for ordered set                                     
// #include <ext/pb_ds/assoc_container.hpp>    
// #include <ext/pb_ds/tree_policy.hpp>     
// using namespace __gnu_pbds;
// // typedef tree<pair<int, int>, null_type, less<pair<int, int>>, rb_tree_tag, tree_order_statistics_node_update> ordered_set;   // for pair
// typedef tree<int , null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> ordered_set;

//  ->uncomment for unordered map of pairs.

// #include<functional>        
// template <typename T>
// inline void hash_combine(std::size_t &seed, const T &val) {seed ^= std::hash<T>()(val) + 0x9e3779b9 + (seed << 6) + (seed >> 2);}
// template <typename T> inline void hash_val(std::size_t &seed, const T &val) {hash_combine(seed, val);}
// template <typename T, typename... Types>
// inline void hash_val(std::size_t &seed, const T &val, const Types &... args) {hash_combine(seed, val);    hash_val(seed, args...);}
// template <typename... Types>
// inline std::size_t hash_val(const Types &... args) {  std::size_t seed = 0; hash_val(seed, args...);return seed;}

// struct pair_hash
//  {   
//      template <class T1, class T2>
//     std::size_t operator()(const std::pair<T1, T2> &p) const 
//     {
//         return hash_val(p.first, p.second);
//     }
// };


 

#define pb push_back
#define ll long long int
#define SORTA(v) sort(v.begin(),v.end())
#define SORTD(v) sort(v.begin(),v.end(),greater<>())
#define endl '\n'
#define FOR(i,n) for(ll i = 0;i<n;i++)
#define FOR1(i,n) for(ll  i=1;i<=n;i++)
#define FAST ios_base::sync_with_stdio(false); cin.tie(NULL);
#define REVERSE(s) reverse(s.begin(),s.end())
#define GCD(a,b) __gcd(a,b)
#define VFIND(v1,val) find(v1.begin() , v1.end() , val )
#define VFILL(v1 , val)   std::fill(v1.begin(), v1.end(), val);
#define ERASE(x) x.erase(unique(x.begin(),x.end()),x.end())

const int MOD = 1e9 + 7;
long long int INF = LLONG_MAX;




void DPM(map<ll,ll>m1)
{
    for(auto it = m1.begin();it!=m1.end();it++)
    {
        cout<<it->first<<" "<<it->second<<endl;
    }
    cout<<endl;
}
void DPV(vector<ll>v)
{
    FOR(i,v.size())cout<<v[i]<<" ";cout<<endl;
}
void DPS(set<ll>s1)
{
    for(auto it =s1.begin();it!=s1.end();it++)cout<<*it<<" ";cout<<endl;
}



ll dx[4] = { 0,0,1,-1 };
ll dy[4] = { 1,-1,0,0 };
string pos = "RLDU";

// check the limits dummy
const ll  NN = 1* (1e5) + 100;

vector< vector<ll> > adj(NN);
vector<ll>child(NN) , vis(NN);
ll n , centroid_1 , centroid_2;

void Dfs(ll v )
{
    if( vis[v] )return;
    vis[v] = child[v] = 1;
    bool is_centroid = true;

    for(auto u : adj[v] )
    {
        if(!vis[u] )
        {
            Dfs(u);
            child[v] += child[u];
            if( child[u] > n/2 )is_centroid = false;
        }
    }
    if(n - child[v] > n/2 )
    {
        is_centroid = false;
    }

    if(is_centroid)
    {
        if(centroid_1 == -1)centroid_1 = v;
        else centroid_2 = v;
    }
}

int main()
{
    FAST;
    ll i ,j , TC = 1;
    cin>>TC;
    FOR1(T,TC)
    {
        cin>>n;
        FOR1(i,n-1)
        {
            ll a,b; cin>>a>>b;
            adj[a].pb(b);
            adj[b].pb(a);
        }
        centroid_1 = centroid_2 = -1;
        Dfs(1);

        if(centroid_2 == -1)
        {
            cout<<centroid_1<<" "<<adj[ centroid_1][0]<<endl;
            cout<<centroid_1<<" "<<adj[ centroid_1][0]<<endl;
        }
        else
        {
            if( adj[centroid_1].size() < adj[centroid_2].size() )
            {
                swap(centroid_1 , centroid_2);
            }
            int x;

            for(auto u : adj[centroid_1] )
            {
                if(u != centroid_2 )
                {
                    x = u;
                    break;
                }
            }
            cout<<centroid_1<<" "<<x<<endl;
            cout<<centroid_2<<" "<<x<<endl;
        }

        FOR1(i,n)adj[i].clear() , child[i] = 0 , vis[i] = 0;

        

        
        
        
    }
}  







/* Note that finding centroids != Finding roots with min heigt..
For min heights see leetcode ques.
5
1 3
2 3     -> work this example out.
4 1
5 1
*/































// vector<vector<int>> vec( n , vector<int> (m, 0));  2d vector 
// to_string(a)
//  s1.substr(1,3) 3 char of s1 starting from 1   s1.sub(pos) starting from pos
// assic value of ('0'-'9') is(48 - 57) and (a-z) is (97-122) and (A-Z) is(65-90) and 32 for space
// reverse(v1.begin() + a , v1.begin() + b) reverse starting from a to b-1
// A lambda function is written like   [](argument1,argument2,.....){//code}
// to create unordered map for pairs unordered_map<pair<ll, ll>, ll, pair_hash> slopeCount;
// unordered_set<pair<ll, ll>, pair_hash> seen;