Problem:
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A Point in a Triangle

There is a triangle formed by three points $(x_1, y_1)$, $(x_2, y_2)$, $(x_3, y_3)$ on a plain.

Write a program which prints "YES" if a point $P$ $(x_p, y_p)$ is in the triangle and "NO" if not.

Input

Input consists of several datasets. Each dataset consists of:

$x_1$ $y_1$ $x_2$ $y_2$ $x_3$ $y_3$ $x_p$ $y_p$

All the input are real numbers. Input ends with EOF. The number of datasets is less than or equal to 100.

Constraints

You can assume that:

$ -100 \leq x_1, y_1, x_2, y_2, x_3, y_3, x_p, y_p \leq 100$

1.0 $\leq$ Length of each side of a tringle

0.001 $\leq$ Distance between $P$ and each side of a triangle

Output

For each dataset, print "YES" or "NO" in a line.

Sample Input

0.0 0.0 2.0 0.0 2.0 2.0 1.5 0.5
0.0 0.0 1.0 4.0 5.0 3.0 -1.0 3.0

Output for the Sample Input

YES
NO

Solution in C# (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace OJcsharp
{
    class Program
    {
        static bool sannkaku(string s)
        {
            string[] str = s.Split(' ');
            double x1 = double.Parse(str[0]), y1 = double.Parse(str[1]), x2 = double.Parse(str[2]), y2 = double.Parse(str[3]), x3 = double.Parse(str[4]), y3 = double.Parse(str[5]), xp = double.Parse(str[6]), yp = double.Parse(str[7]);
            double z1 = (x3 - x2) * (yp - y2) - (y3 - y2) * (xp - x2), z2 = (x1 - x3) * (yp - y3) - (y1 - y3) * (xp - x3), z3 = (x2 - x1) * (yp - y1) - (y2 - y1) * (xp - x1);
            if ((z1 > 0 && z2 > 0 && z3 > 0) || (z1 < 0 && z2 < 0 && z3 < 0))
                return true;
            else
                return false;
        }
        static void Main(string[] args)
        {
            bool[] ans = new bool[100];
            int count = 0;
            string str=Console.ReadLine();
            do
            {
                ans[count] = sannkaku(str);
                str=Console.ReadLine();
                count++;
            } while (String.IsNullOrEmpty(str) == false);
            for (int n = 0; n < count; n++)
            {
                if (ans[n] == true)
                    Console.WriteLine("YES");
                else
                    Console.WriteLine("NO");
            }
        }
    }
}