51NOD1264線段相交

題目鏈接:點擊打開鏈接

解題思路:

　　用直線和點的關係來判斷.把A和B這條線段看成一條直線,分別看C和D關於這條直線的位置關係,如果一正一負,那麼必然相交.

完整代碼:

#include <algorithm>
#include <iostream>
#include <cstring>
#include <climits>
#include <cstdio>
#include <string>
#include <cmath>
#include <set>
#include <queue>
#include <map>
#include <vector>
#include <cstdlib>
#include <stack>
#include <time.h>
using namespace std;
typedef long long LL;
const int MOD = int(1e9)+7;
const int INF = 0x3f3f3f3f;
const double EPS = 1e-9;
const double PI = acos(-1.0); //M_PI;
const int maxn = 100001;

struct point
{
        double x , y;
        point(double a , double b) : x(a) , y(b) {};
        point() {};
        void input()
        {
                scanf("%lf%lf",&x,&y);
        }
};

bool line_make_point(point a , point b , point c , point d)
{
        double C = (c.y - a.y) * (a.x - b.x) - (a.y - b.y) * (c.x - a.x);
        double D = (d.y - a.y) * (a.x - b.x) - (a.y - b.y) * (d.x - a.x);
        if(C * D > 0)   return false;
        return true;
}

bool check(point a , point b , point c , point d)
{
        if(!line_make_point(a , b , c , d))     return false;
        if(!line_make_point(c , d , a , b))     return false;
        return true;
}

int main()
{
#ifdef DoubleQ
    freopen("in.txt","r",stdin);
#endif
    int T;
    scanf("%d",&T);
    while(T--)
    {
            struct point a , b , c , d;
            a.input();
            b.input();
            c.input();
            d.input();
            if(check(a , b , c , d))
                printf("Yes\n");
            else
                printf("No\n");
    }
    return 0;
}


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