ACM题（凸包），为什么两端代码差不多却时间差距这么大？

welon123 2010-11-22 08:38:08

http://acm.hdu.edu.cn/showproblem.php?pid=2202

为什么下边两段代码，第一段是别人的，提交只有31MS，我的却是2000MS，超时
即使我改成按海伦公式算面积，仍然是2000，时间差距主要在哪里呀

#include <iostream>

#include <cmath>

#include <stdlib.h>

using namespace std;

typedef struct

{

    int x;

    int y;

}point;



point result[50001];

point m[50001];

int n,top;



double dis(point a, point b)

{

    double distance;

    

    distance = (a.x - b.x) * (a.x - b.x) + (a.y - b.y) * (a.y - b.y);

    distance = sqrt(distance);

    

    return distance;

}



double Multiply(point a, point b, point c)

{

    return (b.x - a.x) * (c.y - a.y) - (b.y - a.y) * (c.x - a.x);//计算叉积

}



int cmp(const void *a, const void *b)

{

    point *c, *d;

    double t;

    

    c = (point *)a;

    d = (point *)b;

    t = Multiply(m[0],*c,*d);

    if(t < 0)

        return 1;

    else if(t == 0 && (dis(m[0],*c) < dis(m[0],*d)))

        return 1;

    else

        return -1;

}



void tubao()

{

    int i;

    result[0] = m[0];

    result[1] = m[1];

    result[2] = m[2];

    top = 2;

    for(i = 3; i <= n; i ++)

    {

        while(Multiply(result[top - 1],result[top], m[i]) <= 0 && top >= 2)//叉积为负 向右拐 即凹；

        {                                                                 //叉积为正 向左拐 即凸.

            top --;

        }

        result[++ top] = m[i];

    }

}



int main()

{

    int i,j,k,px,py,p;

    point temp;

    double a,b,c,s;

    while(scanf("%d",&n)!=EOF)

    {

        for(i=0;i<n;i++)

        {

            scanf("%d%d",&m[i].x,&m[i].y);

            if(i == 0 || m[i].y < py)           

            {

                px = m[i].x;

                py = m[i].y;

                p = i;

            }

            else if(m[i].y == py && m[i].x < px)

            {

                px = m[i].x;

                py = m[i].y;

                p = i;

            } 

        }

        temp=m[0];m[0]=m[p];m[p]=temp;

        qsort(m+1,n-1,sizeof(m[0]),cmp);

        m[n]=m[0];

        tubao();

        double area[50001];

        double max =0;

              for(i=0;i<top;i++)       //求面积

            {

                for(j=i+1;j<top;j++)

                {

                    for(k=j+1;k<top;k++)

                    {            

                        a = dis(result[i],result[j]);//海伦公式

                        b = dis(result[i],result[k]);

                        c = dis(result[j],result[k]);

                        s=(a+b+c)/2;

                        a=s-a;

                        b=s-b;

                        c=s-c;

                        area[i]=s*a*b*c;

                        area[i]=sqrt(area[i]);

                        if(max<area[i])

                            max=area[i];

                    }

                }

            }

            printf("%.2lf\n",max);

    }

    return 0;

}

我的代码：

#include<iostream>

#include<cmath>

#define SIZE 50010

using namespace std;



int N;

typedef struct

{

	double x;

	double y;

}Point;

Point p[SIZE], stack[SIZE];

int top;



double Mul(Point p1, Point p2, Point p3)

{

	return (p2.x - p1.x) * (p3.y - p1.y) - (p2.y - p1.y) * (p3.x - p1.x);

}



double Dis(Point a, Point b)

{

	return sqrt((a.x - b.x) * (a.x - b.x) + (a.y - b.y) * (a.y - b.y));

}



int cmp(const void *a, const void *b)

{

	Point c = *(Point *)a;

	Point d = *(Point *)b;

	double k = Mul(p[0], c, d);

	if(k < 0 || (!k && Dis(c, p[0]) > Dis(d, p[0])))

		return 1;

	return -1;

}



void Convex()

{

	for(int i = 1; i < n; i++)

	{

		Point temp;

		if(p[i].y < p[0].y || (p[i].y == p[0].y && p[i].x < p[0].x))

		{

			temp = p[i];

			p[i] = p[0];

			p[0] = temp;

		}

	}



	qsort(p + 1, n - 1, sizeof(p[0]), cmp);



	stack[0] = p[0];        

	stack[1] = p[1];

	stack[2] = p[2];

	top =2;

	for(int i = 3; i < n; i++)

	{

		while(top >= 2 && Mul(stack[top - 1], stack[top], p[i]) <= 0) 

			top--;

		top++;

		stack[top] = p[i];

	}

}



int main()

{

	int i, j, k;

	while(cin >> N, N){

		for(i = 0; i < N; i++)

			cin >> p[i].x >> p[i].y;



		Convex();



		double max_area = 0;

		double m;

		for(i = 0; i <= top - 2 ; i++)

		{

			for(j = i + 1; j <= top - 1; j++)

				for(k = j + 1; k <= top; k++)

				{

					m = fabs(Mul(stack[i], stack[j], stack[k]) / 2);

					max_area = m > max_area ? m : max_area;

				}

		}

		cout.setf(ios::fixed);

		cout.precision(2);

		cout << max_area << endl;

	}

	return 0;

}