Language: C++

#include <bits/stdc++.h>
#define fi first
#define se second
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define rrep(i,n) for(int i = 1; i <= (n); ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
#define srep(i,s,t) for (int i = s; i < t; ++i)
#define rng(a) a.begin(),a.end()
#define maxs(x,y) (x = max(x,y))
#define mins(x,y) (x = min(x,y))
#define limit(x,l,r) max(l,min(x,r))
#define lims(x,l,r) (x = max(l,min(x,r)))
#define isin(x,l,r) ((l) <= (x) && (x) < (r))
#define pb push_back
#define sz(x) (int)(x).size()
#define pcnt __builtin_popcountll
#define uni(x) x.erase(unique(rng(x)),x.end())
#define snuke srand((unsigned)clock()+(unsigned)time(NULL));
#define show(x) cout<<#x<<" = "<<x<<endl;
#define PQ(T) priority_queue<T,v(T),greater<T> >
#define bn(x) ((1<<x)-1)
#define dup(x,y) (((x)+(y)-1)/(y))
#define newline puts("")
#define v(T) vector<T>
#define vv(T) v(v(T))
using namespace std;
typedef long long int ll;
typedef unsigned uint;
typedef unsigned long long ull;

inline int in() { int x; scanf("%d",&x); return x;}
template<typename T>inline istream& operator>>(istream&i,v(T)&v)
{rep(j,sz(v))i>>v[j];return i;}
template<typename T>string join(const v(T)&v)
{stringstream s;rep(i,sz(v))s<<' '<<v[i];return s.str().substr(1);}
template<typename T>inline ostream& operator<<(ostream&o,const v(T)&v)
{if(sz(v))o<<join(v);return o;}
template<typename T1,typename T2>inline istream& operator>>(istream&i,pair<T1,T2>&v)
{return i>>v.fi>>v.se;}
template<typename T1,typename T2>inline ostream& operator<<(ostream&o,const pair<T1,T2>&v)
{return o<<v.fi<<","<<v.se;}
template<typename T>inline ll suma(const v(T)& a) { ll res(0); for (auto&& x : a) res += x; return res;}
const double eps = 1e-10;
const ll LINF = 1001002003004005006ll;
//const int INF = 1001001001;
#define dame { puts("-1"); return 0;}
#define yn {puts("Second");}else{puts("First");}
const int MX = 200005;
#define RET(ans) {cout<<ans<<endl;return 0;} 

    // 二次元ベクターの基本
    /*
    vector<vector<int>> dp; // 宣言
    dp.resize(n); // 1次元めの要素数決定
    dp[i].push_back(int); // プッシュバック
    rep(i,n){
        sort(dp[i].begin(),dp[i].end()); // 二次元めを昇順ソート
    }
    */

    // 整数スキャン（複数）
    /*
    int x,y,z;
    scanf("%d%d%d",&x,&y,&z);

    // n個の整数のスキャン
    /*
    ll a[n] = {};
    rep(i,n){
        scanf("%lld",&a[i]);
    }
    */

    // 文字列スキャン
    /*
    string s; cin >> s;
    */

    // n個の文字列スキャン
    /*
    vector<string> slist;
    rep(i,n){
        string s; cin >> s;
        slist.push_back(s);
    }
    */

// n要素で初期化
void init(int par[], int rank[], int n){
    for(int i=0;i<n;i++){
        par[i] = i;
        rank[i] = 0;
    }
}

// 木の根を求める
int find(int par[], int x){
    if(par[x] == x){
        return x;
    }else{
        return par[x] = find(par,par[x]);
    }
}

// xとyの属する集合を併合
void unite(int par[], int rank[], int x, int y){
    x = find(par,x);
    y = find(par,y);
    if(x == y) return;

    if(rank[x] < rank[y]){
        par[x] = y;
    }else{
        par[y] = x;
        if(rank[x] == rank[y]) rank[x]++;
    }
}

// xとyが同じ集合に属するか否か
bool same(int par[], int x, int y){
    return find(par,x) == find(par,y);
}



    
struct edge{
    int to, cost;
};
typedef pair<int, int> P; // firstは最短距離、secondは頂点の番号

void dijkstra(int s, int V, int INF, int d[], vector<struct edge>& G[]){
    // greater<P>を指定することでfirstが小さい順に取り出せるようにする
    priority_queue<P, vector<P>, greater<P> > que;
    fill(d, d+V, INF);
    d[s] = 0;
    que.push(P(0, s));

    while(!que.empty()){
        P p = que.top();
        que.pop();
        int v = p.second;
        for(int i=0; i<G[v].size(); i++){
            edge e = G[v][i];
            if(d[e.to] > d[v] + e.cost){
                d[e.to] = d[v] + e.cost;
                que.push(P(d[e.to] e.to));
            }
        }
    }
}


int cmp1(const void *a, const void *b){ // 昇順
    // qsort
    // http://www.cc.kyoto-su.ac.jp/~yamada/ap/qsort.html

    return *(int*)a - *(int*)b;
}
int cmp2(const void *a, const void *b){ // 降順
    // qsort
    // http://www.cc.kyoto-su.ac.jp/~yamada/ap/qsort.html

    return *(int*)b - *(int*)a;
}

int main() {
    int n;
    cin >> n;

    
    // main function
    int INF = 1001001001;
    int V = n;
    vector<struct edge> G[V];
    int d[V];

    int par[n];
    int rank[n];

    init(par,rank,n);

    int a[n-1][2];

    rep(i,n-1){
        cin >> a[i][0] >> a[i][1];
        a[i][0]--;
        a[i][1]--;
        if(a[i][0]>a[i][1])swap(a[i][0],a[i][1]);
        unite(par,rank,a[i][0],a[i][1]);
        edge e1; e1.to = a[i][1]; e1.cost = 1;
        G[a[i][0]].push_back(e1);
        edge e2; e2.to = a[i][0]; e2.cost = 1;
        G[a[i][1]].push_back(e2);
    }
    qsort(a, n, sizeof(a[0]), cmp1);



    int x = 0;
    int ra = 0;
    rep(i,n){
        //cout << rank[i] << endl;
        if(ra<rank[i]){
            x = i;
            ra = rank[i];
        }
    }

    // diikstra

    dijkstra(x, V,  INF, d, G);
    int co = 0;
    rep(i,n){
        //cout << d[i] << endl;
        co = max(co,d[i]);
    }
    //cout << co;
    if(co%3==1)yn;

    return 0;
}
 
 


Compiler output:
a.cc:132:68: error: declaration of 'G' as array of references
  132 | void dijkstra(int s, int V, int INF, int d[], vector<struct edge>& G[]){
      |                                                                    ^
a.cc: In function 'void dijkstra(...)':
a.cc:135:10: error: 'd' was not declared in this scope
  135 |     fill(d, d+V, INF);
      |          ^
a.cc:135:15: error: 'V' was not declared in this scope
  135 |     fill(d, d+V, INF);
      |               ^
a.cc:135:18: error: 'INF' was not declared in this scope; did you mean 'LINF'?
  135 |     fill(d, d+V, INF);
      |                  ^~~
      |                  LINF
a.cc:136:7: error: 's' was not declared in this scope
  136 |     d[s] = 0;
      |       ^
a.cc:143:24: error: 'G' was not declared in this scope
  143 |         for(int i=0; i<G[v].size(); i++){
      |                        ^