Lưu ý: Các code mẫu dưới đây chỉ mang tính tham khảo và có thể không AC được bài tập này

Code mẫu của flashmt

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <utility>
using namespace std;

int n,f[10100];
short d[50000000];
pair <int,int> a[400200];

int main()
{
    int m,M=1;
    cin >> n >> m;
    for (int i=0;i<m;i++) 
    {
        scanf("%d%d",&a[i].first,&a[i].second); 
        a[m+i].first=a[i].second;
        a[m+i].second=a[i].first;
    }

    for (int i=2;i<=n;i++) f[i]=f[i-1]+n-i+1;
    sort(a,a+m*2);
    for (int i=1;i<m*2;i++)
        if (a[i]!=a[i-1]) a[M++]=a[i];
    for (int i=0,j=0;i<M;i=j)
    {
        while (a[++j].first==a[i].first);
        for (int ii=i;ii<j;ii++)
            if (a[ii].second!=a[i].first)
                for (int jj=ii+1;jj<j;jj++)
                {
                    int id=f[a[ii].second]+a[jj].second-a[ii].second;
                    if (d[id]) 
                    {
                        cout << d[id] << ' ' << a[ii].second << ' ' << a[i].first << ' ' << a[jj].second << endl;
                        return 0;
                    }
                    d[id]=short(a[i].first);
                }
    }
    puts("-1");
}

Code mẫu của ladpro98

#include <cstring>
#include <vector>
#include <list>
#include <map>
#include <set>
#include <queue>
#include <deque>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <climits>
#include <cstdlib>
#include <ctime>
#include <memory.h>
#include <cassert>
#include <climits>
#define FOR(i, a, b) for(int i = (a); i < (b); i++)
#define REP(i, a, b) for(int i = (a); i <=(b); i++)
#define FORD(i, a, b) for(int i = (a); i > (b); i--)
#define REPD(i, a, b) for(int i = (a); i >=(b); i--)
#define TR(it, a) for(typeof((a).begin()) it = (a).begin(); it != (a).end(); it++)
#define RESET(a, v) memset((a), (v), sizeof((a)))
#define SZ(a) (int((a).size()))
#define ALL(a) (a).begin(), (a).end()
#define PB push_back
#define MP make_pair
#define LL long long
#define LD long double
#define II pair<int, int>
#define X first
#define Y second
#define VI vector<int>
const int N = 10005;
using namespace std;
int n, m;
VI a[N];
II cnt[N];
int adj[N];
int per[N];

int main() {
    ios :: sync_with_stdio(0); cin.tie(0);
    cin >> n >> m;
    int u, v;
    FOR(i, 0, m) {
        cin >> u >> v;
        a[u].PB(v); a[v].PB(u);
    }
    REP(i, 1, n) {
        sort(ALL(a[i]));
        a[i].resize(unique(ALL(a[i])) - a[i].begin());
    }
    REP(i, 1, n) per[i] = i;
    random_shuffle(per + 1, per + 1 + n);
    REP(i, 1, n) {
        int u = per[i];
        int top = 0;
        TR(it, a[u]) TR(v, a[*it])
        if (u != *v) {
            adj[top++] = *v;
            if (++cnt[*v].Y == 2) {
                cout << u << ' ' << *it << ' ' << *v << ' ' << cnt[*v].X;
                return 0;
            }
            else cnt[*v].X = *it;
        }
        FOR(i, 0, top)
            cnt[adj[i]].Y = 0;
    }
    cout << -1;
    return 0;
}

Code mẫu của RR

#include <sstream>
#include <iomanip>
#include <iostream>
#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <cmath>
#include <algorithm>
#include <vector>
#include <set>
#include <map>
#include <stack>
#include <queue>
#include <string>
#include <deque>
#include <complex>
#include <ctime>

#define FOR(i,a,b) for(int i=(a),_b=(b); i<=_b; i++)
#define FORD(i,a,b) for(int i=(a),_b=(b); i>=_b; i--)
#define REP(i,a) for(int i=0,_a=(a); i<_a; i++)
#define FORN(i,a,b) for(int i=(a),_b=(b);i<_b;i++)
#define DOWN(i,a,b) for(int i=a,_b=(b);i>=_b;i--)
#define SET(a,v) memset(a,v,sizeof(a))
#define sqr(x) ((x)*(x))
#define ll long long
#define F first
#define S second
#define PB push_back
#define MP make_pair

#define DEBUG(x) cout << #x << " = "; cout << x << endl;
#define PR(a,n) cout << #a << " = "; FOR(_,1,n) cout << a[_] << ' '; cout << endl;
#define PR0(a,n) cout << #a << " = "; REP(_,n) cout << a[_] << ' '; cout << endl;
using namespace std;

//Buffer reading
int INP,AM,REACHEOF;
#define BUFSIZE (1<<12)
char BUF[BUFSIZE+1], *inp=BUF;
#define GETCHAR(INP) { \
    if(!*inp) { \
        if (REACHEOF) return 0;\
        memset(BUF,0,sizeof BUF);\
        int inpzzz = fread(BUF,1,BUFSIZE,stdin);\
        if (inpzzz != BUFSIZE) REACHEOF = true;\
        inp=BUF; \
    } \
    INP=*inp++; \
}
#define DIG(a) (((a)>='0')&&((a)<='9'))
#define GN(j) { \
    AM=0;\
    GETCHAR(INP); while(!DIG(INP) && INP!='-') GETCHAR(INP);\
    if (INP=='-') {AM=1;GETCHAR(INP);} \
    j=INP-'0'; GETCHAR(INP); \
    while(DIG(INP)){j=10*j+(INP-'0');GETCHAR(INP);} \
    if (AM) j=-j;\
}
//End of buffer reading

const long double PI = acos((long double) -1.0);
const int MN = 10111;
const int MM = 200111;

int n, m;
vector<int> ke[MN];
map< pair<int,int>, int > two;

int main() {
    clock_t start = clock();
    scanf("%d%d", &n, &m);
    FOR(i,1,m) {
        int u, v; scanf("%d%d", &u, &v);
        ke[u].PB(v);
        ke[v].PB(u);
    }

    FOR(i,1,n) {
        sort(ke[i].begin(), ke[i].end());
        ke[i].resize(unique(ke[i].begin(), ke[i].end()) - ke[i].begin());
    }

    FORD(a,n,1) {
        if (clock() - start > 2 * CLOCKS_PER_SEC) {
            cout << -1 << endl;
            return 0;
        }
        if (ke[a].size() * ke[a].size() > 40111000) continue;
        REP(i,ke[a].size())
        FOR(j,i+1,ke[a].size()-1) {
            int b = ke[a][i], c = ke[a][j];
            if (two.find(MP(b,c)) != two.end()) {
                cout << a << ' ' << b << ' ' << two[MP(b,c)] << ' ' << c << endl;
                return 0;
            }
            two[MP(b,c)] = a;
            two[MP(c,b)] = a;
        }
    }
    cout << -1 << endl;
    return 0;
}

Code mẫu của hieult

#include<cstdio>
#include<cmath>
#include<math.h>
#include<cstring>
#include<cstdlib>
#include<cassert>
#include<ctime>
#include<algorithm>
#include<iterator>
#include<iostream>
#include<cctype>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<queue>
#include<list>
#define ep 0.00001
#define maxn 10000
#define oo 2000000001
#define modunlo 111539786
#define TR(c, it) for(typeof((c).begin()) it=(c).begin(); it!=(c).end(); it++)
#define fi first
#define se second
//#define g 9.81
double const PI=4*atan(1.0);

using namespace std;

typedef pair<int, int> II;
typedef vector<int> VI;
typedef vector<II> VII;
typedef vector<VI> VVI;
typedef vector<VII> VVII;

int n,m,a,b;
vector<vector<int> > V;
bool A[maxn + 2][maxn + 2] = {0};


bool duyet(int x){
    int f[maxn + 3] = {0};
    TR(V[x], it){
        int u = *it;
        TR(V[u], it){
            f[*it]++;
            if( (*it) != x && f[*it] > 1){
                int i = *it;
                memset(f,0,sizeof(f));
                TR(V[x], it) f[*it]++;
                TR(V[i], it) f[*it]++;
                int a = -1, b = -1;
                for(int j = 1; j <= n; j++){
                    if(f[j] > 1){
                        if(a == -1) a = j;
                        else{
                            printf("%d %d %d %d",x,a,i,j);
                            break; 
                        }
                    }
                }
                return true;
            }
        }
    }
    return false;
}

int main(){
   // freopen("input.in","r",stdin);
   // freopen("output.out","w",stdout);
    scanf("%d %d",&n,&m);
    V.resize(n+2);
    for(int i = 0 ; i < m; i++){
        scanf("%d %d",&a,&b);
        if(!A[a][b]){
            V[a].push_back(b);
            V[b].push_back(a);
            A[a][b] = 1;
            A[b][a] = 1;
        }
    }
    for(int i = 1; i <= n; i++){
        if(duyet(i)){
            return 0;
        }
    }
    printf("-1");
}