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++: // clang-format off
#include <bits/stdc++.h>
#define int long long
#define main signed main()
// #define main int main()
#define loop(i, a, n) for (int i = (a); i < (n); i++)
#define rep(i, n) loop(i, 0, n)
#define all(v) (v).begin(), (v).end()
#define rall(v) (v).rbegin(), (v).rend()
#define prec(n) fixed << setprecision(n)
#define stlice(from, to) substr(from, (to) - (from) + 1)
#define pb push_back
#define mp make_pair
#define mt make_tuple
#define fi first
#define se second
using namespace std;
using pii = pair<int, int>;
using vi = vector<int>;
using vd = vector<double>;
using vc = vector<char>;
using vb = vector<bool>;
using vs = vector<string>;
using vpii = vector<pii>;
using vvi = vector<vi>;
using vvb = vector<vb>;
using vvpii = vector<vpii>;
template<typename A> using fn = function<A>;
constexpr int INF = sizeof(int) == sizeof(long long) ? 1000000000000000000LL : 1000000000;
constexpr int MOD = 1000000007;
constexpr double PI = acos(-1);
template<typename A, typename B> bool cmin(A &a, const B &b) { return a > b ? (a = b, true) : false; }
template<typename A, typename B> bool cmax(A &a, const B &b) { return a < b ? (a = b, true) : false; }
constexpr bool odd(const int &n) { return n & 1; }
constexpr bool even(const int &n) { return !odd(n); }
template<typename V> constexpr typename V::value_type sum(const V &v) { return accumulate(all(v), 0); }
void solve();
main { solve(); return 0; }
// clang-format on

bool existIntersection(int x1, int y1, int x2, int y2, int x3, int y3, int x4, int y4) {
  return ((x1 - x2) * (y3 - y1) + (y1 - y2) * (x1 - x3)) *
           ((x1 - x2) * (y4 - y1) + (y1 - y2) * (x1 - x4)) <
         0;
}

void solve() {
  double x1, y1, x2, y2, x3, y3, xp, yp;
  while (cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> xp >> yp) {
    if ((x1 - x3) * (y1 - y2) == (x1 - x2) * (y1 - y3)) {
      cout << "NO" << endl;
      break;
    }
    if (existIntersection(x1, y1, x2, y2, x3, y3, xp, yp)) {
      cout << "NO" << endl;
      break;
    }
    if (existIntersection(x1, y1, x3, y3, x2, y2, xp, yp)) {
      cout << "NO" << endl;
      break;
    }
    if (existIntersection(x1, y1, xp, yp, x2, y2, x3, y3)) {
      cout << "NO" << endl;
      break;
    }
    cout << "YES" << endl;
  }
}

C++: #include<iostream>
#include<algorithm>

using namespace std;
#define EPS (1e-10)
#define ABS(x) ((x)<0?-(x):(x))
struct L{
    L(double xa, double ya, double xb, double yb){
	x1 = xa;
	y1 = ya;
	x2 = xb;
	y2 = yb;
	if(xa-xb == 0){
	    a = 1;
	    b = 0;
	    c = -xa;
	}
	else {
	    a = (ya-yb);
	    b = (xb-xa);
	    c = -(a*xa + b*ya);
	}
    };
    double f(double x, double y){
	return a*x + b*y + c;
    }
    double a, b, c, x1, x2, y1, y2;//0 = ax + by + c [x1, x2]
};

bool cross(L a, L b){
    return (a.f(b.x1, b.y1) * a.f(b.x2, b.y2) <= 0 && 
	    b.f(a.x1, a.y1) * b.f(a.x2, a.y2) < 0);
}

double x1, y1, x2, y2, x3, y3, xp, yp;

int main(){
    while(cin){
	cin >> x1 >> y1 >> x2 >> y2>> x3 >> y3 >> xp >> yp;
	L a = L(x1, y1, x2, y2);
	L b = L(x1, y1, x3, y3);
	L c = L(x2, y2, x3, y3);
	L p = L(xp, yp, 1000, 2003.141592);
	int res = 0;
	if(cross(a,p))res++;
	if(cross(b,p))res++;
	if(cross(c,p))res++;
	if(res == 1)cout << "YES" <<  endl;
	else cout << "NO"<<endl;
    }
    return 0;
}

C++: /* {{{ Shinobu kawaii */
/*
  ______   __        _                  __                 
.' ____ \ [  |      (_)                [  |                
| (___ \_| | |--.   __   _ .--.   .--.  | |.--.   __   _   
 _.____`.  | .-. | [  | [ `.-. |/ .'`\ \| '/'`\ \[  | | |  
| \____) | | | | |  | |  | | | || \__. ||  \__/ | | \_/ |, 
 \______.'[___]|__][___][___||__]'.__.'[__;.__.'  '.__.'_/ 

*/
/* }}} */

#include <bits/stdc++.h>
using namespace std;

// #define int long long

/* cpp template {{{ */

/* short */
#define pb push_back
#define mp make_pair
#define Fi first
#define Se second
#define ALL(v) (v).begin(), (v).end()

/* REPmacro */
#define REPS(i, a, n) for (int i = (a); i < (n); ++i)
#define REP(i, n) REPS(i, 0, n)
#define RREP(i, n) REPS(i, 1, n + 1)
#define DEPS(i, a, n) for (int i = (a); i >= n; --i)
#define DEP(i, n) DEPS(i, n, 0)

/* typedef */
typedef long long ll;
typedef unsigned long long ull;
typedef vector<int> vi;
typedef vector<vector<int> > vii;
typedef pair<int, int> pii;

/* const */
const int INF = 999999999;
const double EPS = 1e-9;
const int dx[] = {0, 1, 0, -1, 1, -1, 1, -1}, dy[] = {1, 0, -1, 0, 1, -1, -1, 1};

/* func */
inline bool inside(int y, int x, int H, int W) {return y >= 0 && x >= 0 && y < H && x < W;}
inline int in() {int x; std::cin >> x; return x;}
template <typename T>
void print(std::vector<T>& v, char c = ' ') {
  REP(i, v.size()) {
    if (i != 0) std::cout << c;
    std::cout << v[i];
  }
  std::cout << '\n';
}
template <typename T>
void print(T x) {
  std::cout << x << '\n';
}

/* }}} */

typedef pair<double, double> P;

double cross(P& a, P& b) {
  return a.Fi * b.Se - a.Se * b.Fi;
}

P operator +(P& a, P& b) {
  return mp(a.Fi + b.Fi, a.Se + b.Se);
}
P operator -(P& a, P& b) {
  return mp(a.Fi - b.Fi, a.Se - b.Se);
}

signed main()
{
  cin.tie(0);
  ios::sync_with_stdio(false);
  P n[3];
  P p;
  while (cin >> n[0].Fi >> n[0].Se) {
    REPS(i, 1, 3) cin >> n[i].Fi >> n[i].Se;
    cin >> p.Fi >> p.Se;
    bool flg = true;
    P v[] = {n[1] - n[0], n[2] - n[1], n[0] - n[2]};
    REP(i, 3) {
      P t = p - n[i];
      //print(cross(v[i], t));
      if (cross(v[i], t) < EPS) {
        print("NO");
        flg = false;
        break;
      }
    }
    if (flg) print("YES");
  }

  return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;

int main(){

  double x1, y1, x2, y2, x3, y3, x, y;

  while(cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x >> y){

    double a = x1*(y2 - y3) + x2*(y3 - y1) + x3*(y1 - y2);
    double b = x*(y1 - y2) + x1*(y2 - y) + x2*(y - y1);
    double c = x*(y2 - y3) + x2*(y3 - y) + x3*(y - y2);
    double d = x*(y3 - y1) + x3*(y1 - y) + x1*(y - y3);

    if((a<0 && b<0 && c<0 && d<0) || (a>0 && b>0 && c>0 && d>0))
      puts("YES");
    else puts("NO");

  }
}