Problem: 2063C - Remove Exactly Two
Language: C++17 (GCC 7-32)

Previous submission (verdict: Wrong answer on test 2):
#include <bits/stdc++.h>
using namespace std;
 
/* clang-format off */
 
/* TYPES  */
#define ll long long
#define pii pair<int, int>
#define pll pair<long long, long long>
#define vi vector<int>
#define vll vector<long long>
#define mll map<long long, long long>
#define sll set<long long>
#define sc set<char>
#define ff first
#define ss second
 
/* FUNCTIONS */
#define f(i,s,e) for(long long int i=s;i<e;i++)
#define cf(i,s,e) for(long long int i=s;i<=e;i++)
#define rf(i,e,s) for(long long int i=e-1;i>=s;i--)
#define pb push_back
#define eb emplace_back
 
/* PRINTS */
template <class T>
void print_v(vector<T> &v) { cout<<  "{"; for (auto x : v) cout<<  x << ","; cout<<  "\b}"; }
 
/* UTILS */
#define MOD 1000000007
#define PI 3.1415926535897932384626433832795
#define read(type) readInt<type>()
ll min(ll a,int b) { if (a<b) return a; return b; }
ll min(int a,ll b) { if (a<b) return a; return b; }
ll max(ll a,int b) { if (a>b) return a; return b; }
ll max(int a,ll b) { if (a>b) return a; return b; }
ll gcd(ll a,ll b) { if (b==0) return a; return gcd(b, a%b); }
ll lcm(ll a,ll b) { return a/gcd(a,b)*b; }
string to_upper(string a) { for (int i=0;i<(int)a.size();++i) if (a[i]>='a' && a[i]<='z') a[i]-='a'-'A'; return a; }
string to_lower(string a) { for (int i=0;i<(int)a.size();++i) if (a[i]>='A' && a[i]<='Z') a[i]+='a'-'A'; return a; }
bool prime(ll a) { if (a==1) return 0; for (int i=2;i<=round(sqrt(a));++i) if (a%i==0) return 0; return 1; }
void yes() { cout<<"YES\n"; }
void no() { cout<<"NO\n"; }
 
/*  All Required define Pre-Processors and typedef Constants */
typedef long int int32;
typedef unsigned long int uint32;
typedef long long int int64;
typedef unsigned long long int  uint64;
 
 
/* clang-format on */
void solve(){
    ll n;
    cin>>n;
    vector<set<ll>> graph(n);
    f(i,0,n-1){
        ll x,y;
        cin>>x>>y;
        graph[x-1].insert(y-1);
        graph[y-1].insert(x-1);
    }
    ll maxi=0,node=0;
    f(i,0,n){
        if(graph[i].size()>maxi){
            maxi=graph[i].size();
            node=i;
        }
    }
    ll ans=maxi,maxi2=0;
    f(i,0,n){
        if(i!=node&&graph[i].find(node)!=graph[i].end()){
            graph[i].erase(node);
            
        }
        if(i!=node) maxi2=max(maxi2,graph[i].size());
    }
    // if(maxi2==0){
    //     cout<<ans-1<<endl;
    //     return;
    // }
    //cout<<maxi2<<endl;
    cout<<ans+maxi2-1<<endl;
}
 
/* Main()  function */
int main()
{
 
    #ifdef LOCAL
    freopen("input.txt", "rt", stdin);
    freopen("output.txt", "wt", stdout);
    #endif
    ios::sync_with_stdio(0);
    cin.tie(0);
    ll t;
    cin>>t;
    while(t--)
    {
        solve();
    }
    return 0;
}

Submission (verdict: Wrong answer on test 2):
#include <bits/stdc++.h>
using namespace std;

/* clang-format off */
 
/* TYPES  */
#define ll long long
#define pii pair<int, int>
#define pll pair<long long, long long>
#define vi vector<int>
#define vll vector<long long>
#define mll map<long long, long long>
#define sll set<long long>
#define sc set<char>
#define ff first
#define ss second
 
/* FUNCTIONS */
#define f(i,s,e) for(long long int i=s;i<e;i++)
#define cf(i,s,e) for(long long int i=s;i<=e;i++)
#define rf(i,e,s) for(long long int i=e-1;i>=s;i--)
#define pb push_back
#define eb emplace_back
 
/* PRINTS */
template <class T>
void print_v(vector<T> &v) { cout<<  "{"; for (auto x : v) cout<<  x << ","; cout<<  "\b}"; }
 
/* UTILS */
#define MOD 1000000007
#define PI 3.1415926535897932384626433832795
#define read(type) readInt<type>()
ll min(ll a,int b) { if (a<b) return a; return b; }
ll min(int a,ll b) { if (a<b) return a; return b; }
ll max(ll a,int b) { if (a>b) return a; return b; }
ll max(int a,ll b) { if (a>b) return a; return b; }
ll gcd(ll a,ll b) { if (b==0) return a; return gcd(b, a%b); }
ll lcm(ll a,ll b) { return a/gcd(a,b)*b; }
string to_upper(string a) { for (int i=0;i<(int)a.size();++i) if (a[i]>='a' && a[i]<='z') a[i]-='a'-'A'; return a; }
string to_lower(string a) { for (int i=0;i<(int)a.size();++i) if (a[i]>='A' && a[i]<='Z') a[i]+='a'-'A'; return a; }
bool prime(ll a) { if (a==1) return 0; for (int i=2;i<=round(sqrt(a));++i) if (a%i==0) return 0; return 1; }
void yes() { cout<<"YES\n"; }
void no() { cout<<"NO\n"; }
 
/*  All Required define Pre-Processors and typedef Constants */
typedef long int int32;
typedef unsigned long int uint32;
typedef long long int int64;
typedef unsigned long long int  uint64;

/* clang-format on */
void solve()
{
    ll n;
    cin >> n;
    vector<set<ll>> graph(n);
    map<ll, ll> mp;
    f(i, 0, n-1)
    {
        ll x, y;
        cin >> x >> y;
        graph[x - 1].insert(y - 1);
        graph[y - 1].insert(x - 1);
    }
    f(i, 0, n)
    {
        if(mp.find(graph[i].size())==mp.end()){
            mp[graph[i].size()]=0;
        }
        mp[graph[i].size()]++;
    }
    
    ll ans = 0;
    f(i, 0, n)
    {
        ll temp = graph[i].size();
        mp[graph[i].size()]--;
        if (mp[graph[i].size()] == 0)
        {
            mp.erase(graph[i].size());
        }
        for (auto &val : graph[i])
        {
            //cout<<graph[val].size()<<" ";
            mp[graph[val].size()]--;
            if (mp[graph[val].size()] == 0)
            {
                mp.erase(graph[val].size());
            }
            if (mp.find(mp[graph[val].size() - 1]) == mp.end())
            {
                mp[graph[val].size() - 1] = 0;
            }
            mp[graph[val].size() - 1]++;
        }
        if((*mp.rbegin()).first==0){
            ans=max(temp-1,ans);
        }else{
            ans = max(ans, temp + (*mp.rbegin()).first - 1);
        }
        
        //cout<<temp<<" "<<(*mp.rbegin()).first<<endl;
        for (auto &val : graph[i])
        {
            mp[graph[val].size() - 1]--;
            if (mp[graph[val].size()-1] == 0)
            {
                mp.erase(graph[val].size()-1);
            }
            if (mp.find(mp[graph[val].size()]) == mp.end())
            {
                mp[graph[val].size()] = 0;
            }
            mp[graph[val].size()]++;
        }
        if (mp.find(mp[graph[i].size()]) == mp.end())
            {
                mp[graph[i].size() - 1] = 0;
            }
        mp[graph[i].size()]++;
        // for(auto &[key,val]:mp){
        //     cout<<key<<" "<<val<<endl;
        // }
        // cout<<endl;
    }
    cout<<ans<<endl;
}

/* Main()  function */
int main()
{

#ifdef LOCAL
    freopen("input.txt", "rt", stdin);
    freopen("output.txt", "wt", stdout);
#endif
    ios::sync_with_stdio(0);
    cin.tie(0);
    ll t;
    cin >> t;
    while (t--)
    {
        solve();
    }
    return 0;
}