Go: package main

import (
	"bufio"
	"os"
	"strconv"
	"strings"
	"fmt"
)

var scanner = bufio.NewScanner(os.Stdin)

func main() {
	scanner.Scan()
	n, _ := strconv.Atoi(scanner.Text())
	for i := 0; i < n; i++ {
		scanner.Scan()
		input := strings.Split(scanner.Text(), " ")
		buf := make([]float64, len(input))
		for j := range buf {
			buf[j], _ = strconv.ParseFloat(input[j], 64)
		}
		p1 := complex(buf[0], buf[1])
		p2 := complex(buf[2], buf[3])
		p3 := complex(buf[4], buf[5])
		pk := complex(buf[6], buf[7])
		ps := complex(buf[8], buf[9])
		kin := isAtSameSide(p3, pk, []complex128{p1, p2}) &&
			isAtSameSide(p1, pk, []complex128{p2, p3}) &&
			isAtSameSide(p2, pk, []complex128{p3, p1})
		sin := isAtSameSide(p3, ps, []complex128{p1, p2}) &&
			isAtSameSide(p1, ps, []complex128{p2, p3}) &&
			isAtSameSide(p2, ps, []complex128{p3, p1})
		if (kin || sin) && !(kin && sin) {
			fmt.Println("OK")
		} else {
			fmt.Println("NG")
		}
	}
}

func isAtSameSide(a, b complex128, line []complex128) bool {
	sa := (real(line[1]) - real(line[0]))*(imag(a)-imag(line[0]))+
		(imag(line[1])-imag(line[0]))*(real(line[0])-real(a))
	sb := (real(line[1]) - real(line[0]))*(imag(b)-imag(line[0]))+
		(imag(line[1])-imag(line[0]))*(real(line[0])-real(b))
	return (sa > 0 && sb > 0) || (sa < 0 && sb < 0)
}

C++: #include <iostream>
#include <vector>
#include <cmath>

using namespace std;
typedef pair<double, double> P;
const double eps = 1e-9;

double Length(P p1, P p2)
{
	return(sqrt((p1.first - p2.first) * (p1.first - p2.first) + (p1.second - p2.second) * (p1.second - p2.second)));
}

double Helon(P p1, P p2, P p3)
{
	double a = Length(p1, p2);
	double b = Length(p2, p3);
	double c = Length(p3, p1);
	double s = (a + b + c) / 2;
	return(sqrt(s * (s - a) * (s - b) * (s - c)));
}

void solve()
{
	int n;
	cin >> n;
	for(int i = 0; i < n; ++i)
	{
		vector<P> Point(3);
		P pk, ps;
		for(int j = 0; j < 3; ++j)
		{
			cin >> Point[j].first >> Point[j].second;
		}
		cin >> pk.first >> pk.second >> ps.first >> ps.second;
		double S = Helon(Point[0], Point[1], Point[2]);
		double S1 = Helon(Point[0], Point[1], pk);
		S1 += Helon(Point[1], Point[2], pk);
		S1 += Helon(Point[2], Point[0], pk);

		double S2 = Helon(Point[0], Point[1], ps);
		S2 += Helon(Point[1], Point[2], ps);
		S2 += Helon(Point[2], Point[0], ps);

		if(fabs(S - S1) < eps && fabs(S - S2) >= eps || fabs(S - S1) >= eps && fabs(S - S2) < eps)
		{
			cout << "OK" << endl;
		}
		else
		{
			cout << "NG" << endl;
		}
	}
}

int main()
{
	solve();
	return(0);
}

C++: #include <iostream>
#include <iomanip>
#include <complex>
#include <vector>
#include <algorithm>
#include <cmath>
#include <cstdio>
using namespace std;

const double EPS = 1e-10;
const double INF = 1e12;
#define EQ(n,m) (abs((n)-(m)) < EPS)
#define X real()
#define Y imag()

typedef complex<double> P;
typedef vector<P> VP;

double dot(P a, P b){
    return (conj(a)*b).X;
}
double cross(P a, P b){
    return (conj(a)*b).Y;
}
bool in_tri(const P& p, const VP& tri){
    return dot(tri[1]-tri[0], p-tri[0])>0 &&
        dot(tri[2]-tri[1], p-tri[1])>0 &&
        dot(tri[0]-tri[2], p-tri[2])>0;
}

int main(){
    int n;
    cin >> n;
    for(int i=0; i<n; i++){
        VP tri(3);
        for(int j=0; j<3; j++){
            int x,y;
            cin >> x >> y;
            tri[j] = P(x,y);
        }
        VP sg(2);
        for(int j=0; j<2; j++){
            int x,y;
            cin >> x >> y;
            sg[j] = P(x,y);
        }
        if(in_tri(sg[0], tri) != in_tri(sg[1], tri)){
            cout << "OK" << endl;
        }else{
            cout << "NG" << endl;
        }
    }
    return 0;
}

C++: #include<iostream>
#include<vector>
#include<complex>
#include<algorithm>
using namespace std;

#define EPS (1e-10)
#define EQ(a,b) (abs((a)-(b)) < EPS)
#define EQV(a,b) (EQ((a).real(),(b).real()) && EQ((a).imag(),(b).imag()))
typedef complex<double> Po;

Po unit(Po p){return p / abs(p);}
  
Po normal(Po p,int m){
  Po x;
  if(!m)x = p * Po(0,1);
  else x = p * Po(0,-1);
  return x;
}

double dot(Po x,Po y){return x.real()*y.real() + x.imag()*y.imag();}

double cross(Po x,Po y){return x.real()*y.imag() - x.imag()*y.real();}

double area(Po a,Po b,Po c){
  return abs(cross(b-a,c-a)/2);
}

bool inter(Po a,Po b,Po c,Po x){
  if( area(a,b,x) + area(b,c,x) + area(c,a,x) > area(a,b,c) + EPS)return false;
  else return true;
}

class line{
 public:
  Po a,b;

  void set(Po x,Po y){
    a = x;
    b = y;
  }

  bool orth(line l){
    return EQ( dot(a-b,l.a-l.b),0.0);
  }

  bool para(line l){
    return EQ( cross(a-b,l.a-l.b),0.0);
  }

  bool on_line(Po x){
    return EQ( cross(b-a,x-a),0.0);
  }

  double line_dis(Po x){
    return abs(cross(b-a,x-a))/abs(b-a);
  }

  Po line_cp(line l){
    return a + (b-a)*cross(l.b-l.a,l.a-a) /cross(l.b-l.a,b-a);
  }

  bool on_seg(Po x){
    return (abs(a-x) + abs(x-b) < abs(a-b) + EPS);
  }

  double seg_dis(Po x){
    if(dot(b-a,x-a)<EPS)return abs(x-a);
    if(dot(a-b,x-b)<EPS)return abs(x-b);
    return abs(cross(b-a,x-a))/abs(b-a);
  }

  int is_cross(line l){
    return (cross(b-a,l.a-a) * cross(b-a,l.b-a)<EPS) && (cross(l.b-l.a,a-l.a) * cross(l.b-l.a,b-l.a) < EPS);
  }

  Po seg_cp(line l){
    double d = abs(cross(l.b-l.a,a-l.a));
    return a + (b-a)*( d / (d * abs(cross(l.b-l.a,b-l.a))));
  }
};

class polygon{
 public:
  vector<Po> v;
  int o;

  void set(void){
    v.clear();
  }

  void Vadd(Po a){
    if(!v.size())o = 0;
    else if(a.imag() < v[o].imag())o = v.size();
    v.push_back(a);
  }

  void sort(void){
    vector<double> seta;
    Po ot;
    ot.real() = 1.0;
    ot.imag() = 0.0;
    swap(v[0],v[o]);
    o = 0;
    for(int i=1;i<(int)v.size();i++){
      seta.push_back(dot(ot,v[i]-v[0]) / (abs(ot)*abs(v[i]-v[0])) );
    } 
    for(int i=1;i<(int)v.size()-1;i++){
      for(int j=i+1;j<(int)v.size();j++){
	if(seta[i-1] < seta[j-1]){
	  swap(seta[i-1],seta[j-1]);
	  swap(v[i],v[j]);
	}
      }
    }
  }

  bool inter(Po a){
    int c = 0;
    double max = 0;
    Po b;
    line l,s;
    for(int i=0;i<(int)v.size();i++)if(max < v[i].real())max = v[i].real();
    b.real() = max+1;
    b.imag() = a.imag();
    s.set(a,b);
    for(int i=0;i<(int)v.size()-1;i++){
      l.set(v[i],v[i+1]);
      if(s.is_cross(l))c++;
    }
    l.set(v[(int)v.size()-1],v[0]);
    if(s.is_cross(l))c++;
    if(c%2)return true;
    else return false;
  }
};

int main(){
  int n;
  Po p1,p2,p3,pk,ps;
  polygon tri;

  cin >> n;

  for(int i=0;i<n;i++){
    cin >> p1.real() >> p1.imag();
    cin >> p2.real() >> p2.imag();
    cin >> p3.real() >> p3.imag();
    cin >> pk.real() >> pk.imag();
    cin >> ps.real() >> ps.imag();
    /*
    tri.set();
    tri.Vadd(p1);
    tri.Vadd(p2);
    tri.Vadd(p3);
    tri.sort();
    */
    //if(tri.inter(pk) != tri.inter(ps))cout << "OK" << endl;
    if(inter(p1,p2,p3,pk) != inter(p1,p2,p3,ps) )cout << "OK" << endl;
    else cout << "NG" << endl;
  }
  
}

C++: #include <bits/stdc++.h>

using namespace std;

#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define REP(i,n) FOR(i,0,n)

#define EPS (1e-8)
#define EQ(a,b) (abs((a)-(b)) < EPS)
#define EQV(a,b) ( EQ(a.real(), b.real()) && EQ(a.imag(), b.imag()) )

typedef complex<double> p;

double cross(p a, p b) {
	return (a.real() * b.imag() - a.imag() * b.real());
}

int main() {
	int n;
	cin >> n;
	while (n--) {
		p point[5];
		REP(i, 5) {
			double a, b;
			scanf("%lf %lf", &a, &b);
			point[i] = p(a, b);
		}
		int cnt[2] = {};
		
		REP(i, 3) {
			p v[2];
			v[0] = point[(i + 1) % 3] - point[i];
			v[1] = point[3] - point[i];
			if (cross(v[0], v[1]) < -EPS) cnt[0]++;
			
			v[1] = point[4] - point[i];
			if (cross(v[0], v[1]) < -EPS) cnt[1]++;
		}
		bool flag[2];
		REP(i, 2) flag[i] =  cnt[i] == 0;
		if (flag[0] == !flag[1]) puts("OK");
		else puts("NG");
	}
	return 0;
}