Go: package main

import (
        "fmt"
)

func main() {
        var x1, y1, x2, y2, x3, y3, px, py float64
        var oax, oay, obx, oby, opx, opy float64
        var det float64
        var s, t float64

        for {
                _, err := fmt.Scanf("%f %f %f %f %f %f %f %f", &x1, &y1, &x2, &y2, &x3, &y3, &px, &py)
                if err != nil {
                        break
                }
                oax = x2 - x1
                oay = y2 - y1
                obx = x3 - x1
                oby = y3 - y1
                opx = px - x1
                opy = py - y1
                det = oax*oby - obx*oay

                if det == 0 {
                        fmt.Printf("NO\n")
                } else {
                        s = (oby*opx - obx*opy) / det
                        t = (oax*opy - oay*opx) / det

                        if (s+t < 1 && s > 0 && t > 0) {
                                fmt.Printf("YES\n")
                        } else {
                                fmt.Printf("NO\n")
                        }
                }
        }
}

C++: #include <iostream>
#include <iomanip>
#include <vector>
#include <string>
#include <stack>
#include <queue>
#include <deque>
#include <set>
#include <map>
#include <algorithm>
#include <sstream>
#include <cstdlib>
#include <cstring>
#include <functional>
#include <numeric>
#include <cmath>
#include <climits>
#include <limits>
#include <cfloat>
#include <fstream>
using namespace std;

int main()
{
//	cut here before submit 
//	freopen("testcase.pt", "r", stdin);
	string str = "";

	while (getline (cin, str )){
		int i;
		vector <double> tpx (3, 0.);
		vector <double> tpy (3, 0.);
		double px = 0.;
		double py = 0.;
	
		stringstream ss(str);

		for (i = 0; i < 3; ++i){
			double tx,ty;
			ss >> tx >> ty;
			tpx[i] = tx; tpy[i] = ty;
		} // end for
		ss >> px >> py;

		vector <double> a(3, 0.);
		vector <double> b(3, 0.);
		vector <double> c(3, 0.);
/*
		for (i = 0; i < 3; ++i){
			cout << "tp[" << i << "]: ";
			cout << '(' << tpx[i] << ',' << tpy[i] << ')' << endl;
		} // end for
		cout << "p: ";
		cout << '(' << px << ',' << py << ')' << endl;
*/
		int j;
		for (i = 0; i < 3; ++i){
			j = (i + 1) % 3;
			if (tpx[i] == tpx[j]){
				a[i] = 1.0;
				b[i] = 0.0;
				c[i] = -a[i]*tpx[i] - b[i]*tpy[i];
			}else if (tpy[i] == tpy[j]){
				a[i] = 0.0;
				b[i] = 1.0;
				c[i] = -a[i]*tpx[i] - b[i]*tpy[i];
			}else{
				a[i] = (tpy[j] - tpy[i]);
				b[i] = -(tpx[j] - tpx[i]);
 				c[i] = -a[i]*tpx[i] - b[i]*tpy[i];
			} // end if
//			cout << a[i] << ' ' << b[i] << ' ' << c[i] << endl;
		} // end for
		vector <double> d(3, 0.0);

		for (i = 0; i < 3; ++i){
			d[i] = a[i]*px + b[i]*py + c[i];
//			cout << "d[" << i << "]: " << d[i] << endl;
		} // end for

		int plus = 0;
		int minus = 0;
		for (i = 0; i < 3; ++i){
			if (d[i] > 0){
				++plus;
			}else{
				++minus;
			} // end if
		} // end for

		if (plus == 1 && minus == 2){
			cout << "YES" << endl;
		}else{
			cout << "NO" << endl;
		} // end if
			
	} // end for
		
	return 0;
}

C++: #include <cstdio>
#include <cstring>
#include <algorithm>
#include <string>
#include <cmath>

double area(double a, double b, double c)
{
	double s = (a+b+c)/2;
	return sqrt(s*(s-a)*(s-b)*(s-c));
}
double distance(double x1, double y1, double x2, double y2)
{
	return sqrt((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2));
}
int main()
{
	double x[3], y[3];
	double xp, yp;
	while (scanf("%lf %lf %lf %lf %lf %lf %lf %lf"
		, &x[0], &y[0], &x[1], &y[1], &x[2], &y[2], &xp, &yp) != EOF)
	{
		double edge[3];
		for (int i = 0; i < 3; ++i)
			edge[i] = distance(x[i], y[i], x[(i+1)%3], y[(i+1)%3]);
		double correctArea = area(edge[0], edge[1], edge[2]);
		double totalArea = 0;
		for (int i = 0; i < 3; ++i)
		{
			totalArea += area(distance(x[i], y[i], xp, yp)
				, distance(x[(i+1)%3], y[(i+1)%3], xp, yp), edge[i]);
		}
		if (abs(correctArea - totalArea) < 0.0001)
			puts("YES");
		else
			puts("NO");
	}

	return 0;
}

C++: #include <algorithm>
#include <cmath>
#include <climits>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <fstream>
#include <iostream>
#include <list>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <vector>
#include <cassert>
#include <functional>

using namespace std;

#define LOG(...) printf(__VA_ARGS__)
//#define LOG(...)
#define FOR(i,a,b) for(int i=(int)(a);i<(int)(b);++i)
#define REP(i,n) for(int i=0;i<(int)(n);++i)
#define ALL(a) (a).begin(),(a).end()
#define RALL(a) (a).rbegin(),(a).rend()
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())
#define RSORT(c) sort((c).rbegin(),(c).rend())
#define CLR(a) memset((a), 0 ,sizeof(a))

typedef long long ll;
typedef unsigned long long ull;
typedef vector<bool> vb;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<vb> vvb;
typedef vector<vi> vvi;
typedef vector<vll> vvll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;

const int dx[] = { -1, 0, 1, 0 }; const int dy[] = { 0, 1, 0, -1 };

struct pos{
	double x;
	double y;
};
int main() {
	REP(i,100){
		vector<pos> point(4);
		REP(j,4){
			cin >> point[j].x >> point[j].y;
		}
		bool yes=true;
		REP(j, 3){
			double cross = (point[(j + 1) % 3].x - point[(j) % 3].x)*(point[3].x - point[(j) % 3].x) + (point[(j + 1) % 3].y - point[(j) % 3].y)*(point[3].y - point[(j) % 3].y);
			if (cross <= 0){
			yes=false;
			}
		}
		if (yes)
		cout << "YES"<<endl;
		else
		cout << "NO" << endl;
	}
}

C++: #include<iostream>
#include <math.h>

using namespace std;

struct Vector2D{
    double x;
    double y;
};
//???????????????(??????????????¨??????)
#define Vertex2D Vector2D

//?????????????????????(a-b)
Vector2D sub_vector(const Vector2D& a, const Vector2D& b)
{
    Vector2D ret;
    ret.x = a.x - b.x;
    ret.y = a.y - b.y;
    return ret;
}

// ????§???¢??¨?????????????????????(2D?????´???)
// ?????????    0:????§???¢????????´???????????????    1:????§???¢????????´???????????????
int hittest_point_polygon_2d(Vertex2D A, Vertex2D B, Vertex2D C, Vertex2D P) {

    //??????????????¨??????????????????
    //ABC???????§???¢????????????????????§??????????????\...

    Vector2D AB = sub_vector(B, A);
    Vector2D BP = sub_vector(P, B);

    Vector2D BC = sub_vector(C, B);
    Vector2D CP = sub_vector(P, C);

    Vector2D CA = sub_vector(A, C);
    Vector2D AP = sub_vector(P, A);

    //??????    Z?????????????¨????????????°????????§???
    double c1 = AB.x * BP.y - AB.y * BP.x;
    double c2 = BC.x * CP.y - BC.y * CP.x;
    double c3 = CA.x * AP.y - CA.y * AP.x;

    if ((c1 > 0 && c2 > 0 && c3 > 0) || (c1 < 0 && c2 < 0 && c3 < 0)) {
        //????§???¢????????´???????????????
        return 0;
    }

    //????§???¢????????´???????????????
    return 1;

}

int main(){
    Vector2D A;
    Vector2D B;
    Vector2D C;
    Vector2D P;

    while (cin >> A.x >> A.y >> B.x >> B.y >> C.x >> C.y >> P.x >> P.y)
    {
        if (hittest_point_polygon_2d(A, B, C, P) == 0){
            cout << "YES" << endl;
        }
        else
        {
            cout << "NO" << endl;
        }
    }

    return 0;
}

C++: #include <iostream>
using namespace std;
double x[4],y[4],px,py,ax,ay,bx,by;
int main()
{
	while(true)
	{
		for(int i = 1;i<=3;i++){cin >> x[i];cin >> y[i];}
		if(cin.eof())break;
		cin >> px;
		cin >> py;
		double m,n,s;
		ax = x[2]-x[1];
		ay = y[2]-y[1];
		bx = x[3]-x[1];
		by = y[3]-y[1];
		double det = ax*by-ay*bx;
		m = (by*px-bx*py)/det;
		n = (-ay*px+ax*py)/det;
		if(m>0 && n > 0&& (m+n) < 1) cout << "YES" << endl;
		else cout << "NO" << endl;
	}
	return 0;
}