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 <algorithm>
#include <cstdio>
#include <vector>
#include <utility>
#include <cmath>
#define fr(a,b,c) for (a=b;a<=c;a++)
#define frr(a,b,c) for (a=b;a>=c;a--)
#define pii pair<int,int>
#define F first
#define S second
#define z 100
using namespace std;

int n,gcd[111][111],d[222][222],e[222][222];
vector< pii > a;
vector< pii > b[222][222],c[222][222];
long long re;

int main()
{
    int i,j,x,y,A,B;
    fr(i,1,100)
        fr(j,i,100)
            if (i==j) gcd[i][j]=i;
            else gcd[i][j]=gcd[j][i]=gcd[i][j-i];
    cin >> n;
    fr(i,1,n) scanf("%d%d",&x,&y), a.push_back(make_pair(x,y));
    fr(i,0,n-2)
        fr(j,i+1,n-1)
            if (a[i].F!=a[j].F || a[i].S!=a[j].S)
            {
                x=a[i].F+a[j].F+z;
                y=a[i].S+a[j].S+z;
                A=a[i].S-a[j].S;
                B=a[j].F-a[i].F;
                if (!A) d[x][y]++;
                else
                    if (!B) e[x][y]++;
                    else
                    {
                        int k=gcd[abs(A)][abs(B)];
                        A/=k; B/=k;
                        if (A<0) A=-A, B=-B;
                        if (B>0) b[x][y].push_back(make_pair(A,B));
                        else c[x][y].push_back(make_pair(-B,A));
                    }
            }
    fr(x,0,200)
        fr(y,0,200)
        {
            re+=1LL*d[x][y]*e[x][y];
            if (!b[x][y].empty() && !c[x][y].empty())
            {
                sort(b[x][y].begin(),b[x][y].end());
                sort(c[x][y].begin(),c[x][y].end());
                int B=b[x][y].size(),C=c[x][y].size();
                i=0; j=0;
                while (i<B && j<C)
                {
                    if (b[x][y][i]<c[x][y][j]) ++i;
                    else 
                        if (b[x][y][i]>c[x][y][j]) ++j;
                        else
                        {
                            int ii=i,jj=j;
                            while (b[x][y][ii]==b[x][y][i] && ii<B) ++ii;
                            while (c[x][y][jj]==c[x][y][j] && jj<C) ++jj;
                            re+=1LL*(ii-i)*(jj-j);
                            i=ii; j=jj;
                        }
                }
            }
        }
    cout << re << endl;
    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>

#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;
const int BUFSIZE = (1<<12) + 17;
char BUF[BUFSIZE+1], *inp=BUF;
#define GETCHAR(INP) { \
    if(!*inp && !REACHEOF) { \
        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);

struct Point {
    int x, y;

    Point(int x = 0, int y = 0) : x(x), y(y) {}

    Point operator + (Point a) { return Point(x+a.x, y+a.y); }
    Point operator - (Point a) { return Point(x-a.x, y-a.y); }
    Point operator * (int k) { return Point(x*k, y*k); }
    Point operator / (int k) { return Point(x/k, y/k); }

    int operator * (Point a) { return x*a.x + y*a.y; }

    void read() { scanf("%d%d", &x, &y); }
} a[1511];

vector<double> ls[222][222];

int nPoint, now, cnt[1511];
double val[1511];

int main() {
    scanf("%d", &nPoint);
    FOR(i,1,nPoint) {
        a[i].read();
        a[i] = a[i] * 2;
    }
    FOR(i,1,nPoint) FOR(j,i+1,nPoint) {
        Point O = (a[i] + a[j]) / 2;
        ls[O.x + 100][O.y + 100].push_back(atan2(a[i].x - O.x, a[i].y - O.y));
    }

    int res = 0;
    FOR(x,0,200) FOR(y,0,200) {
        int now = 0;
        REP(t,ls[x][y].size()) val[++now] = ls[x][y][t];
        sort(val+1, val+now+1);

        FOR(t,1,now) val[now+t] = val[t] + 2*M_PI;

        // PR(val, now);

        int j = 0; val[0] = -1000;
        FOR(t,1,now+now) {
            if (fabs(val[t] - val[j]) > 1e-6) {
                ++j;
                val[j] = val[t];
                cnt[j] = 1;
            }
            else ++cnt[j];
        }
        now = j;

        // PR(val, now);
        // PR(cnt, now);

        int t2 = 1;
        FOR(t,1,now) {
            if (val[t] > M_PI) break;

            while (t2 < now && val[t2] < val[t] + M_PI / 2 - 1e-6) ++t2;
            if (fabs(val[t2] - val[t] - M_PI / 2) < 1e-6) res += cnt[t] * cnt[t2];
        }
    }
    cout << res << endl;
    return 0;
}

Code mẫu của khuc_tuan

#include <iostream>
#include <vector>
using namespace std;

struct Point {
    int x, y;
    Point() {}
    Point(int x, int y) : x(x), y(y) {}
    Point add(Point p) { return Point(x+p.x,y+p.y); }
    Point sub(Point p) { return Point(x-p.x,y-p.y); }
    Point mul(int k) { return Point(x*k,y*k); }
    Point div(int k) { return Point(x/k,y/k); }
};

int n, result;
Point a[1505];
vector<Point> list[222][222];

bool cmp(Point a, Point b) {
    int t = a.x * (a.y - b.y) - a.y * (a.x - b.x);
    return t < 0;
}

void process(vector<Point> &v) {
    sort( v.begin(), v.end(), cmp);
    int j = 0, k = 0;
    for(int i=0;i<v.size();++i) {
        j >?= i + 1;
        k >?= i + 1;
        while(j<v.size() && v[j].x * v[i].x + v[j].y * v[i].y > 0) ++j;
        while(k<v.size() && v[k].x * v[i].x + v[k].y * v[i].y >= 0) ++k;
        result += k - j;
    }
}

int main() {
    scanf("%d", &n);
    for(int i=0;i<n;++i) scanf("%d%d", &a[i].x, &a[i].y), a[i] = a[i].mul(2);
    for(int i=0;i<n;++i)
        for(int j=i+1;j<n;++j) {
            Point mid = a[i].add(a[j]).div(2);
            Point v = a[i] .sub( mid );
            if(v.y<0) v = v.mul(-1);
            if(v.y==0 && v.x<0) v.x = -v.x;
            list[mid.x+110][mid.y+110].push_back(v);
        }
    for(int i=0;i<222;++i)
        for(int j=0;j<222;++j)
            process(list[i][j]);
    printf("%d\n", result);
    return 0;
}