Poj 2187 Beauty Contest （凸包+旋转卡壳）

来源：互联网发布：缓存数据库编辑：程序博客网时间：2024/05/22 11:59

题目链接：poj 2187

对于这道题，我只能呵呵呵呵。我靠，我在poj上wa了20多次才发现有一种坑爹的情况：所有的点在同一条直线上QAQ!好吧，我是考虑不够完全，但是这是什么情况：数据中有重点!题中说好的No two farms share the same pair of coordinates呢？

贴上我的代码。(我是顺时针转的)

#include<cstdio>#include<cstring>#include<iostream>#include<algorithm>using namespace std;const int maxn=100000+10;int N,top;struct node{int x,y;}p[maxn],point,stack[maxn];inline int read(){int x=0,f=1;char ch=getchar();while(ch<'0'||ch>'9'){if(ch=='-')f=-1;ch=getchar();}while(ch>='0'&&ch<='9'){x=x*10+ch-'0';ch=getchar();}return x*f;}int get_cross(node a,node b,node c){return (b.x-a.x)*(c.y-a.y)-(b.y-a.y)*(c.x-a.x);}int get_dis(node a,node b){return (a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y);}void find_point(){int col=0; point=p[0];for(int i=1;i<N;i++){if(p[i].y<point.y || (p[i].y==point.y && p[i].x<point.x)){col=i; point=p[i];}}swap(p[0],p[col]);}bool cmp(node a,node b){//we sort these points as the clockwise.int t=get_cross(point,a,b);if(t<0)return true;if(t>0)return false;int xx=get_dis(point,a),yy=get_dis(point,b);return xx<yy;}void Graham(){top=1;stack[0]=p[0]; stack[1]=p[1];for(int i=2;i<N;i++){//it rotates as the clockwise.while(top>=1 && get_cross(stack[top-1],stack[top],p[i])>=0)top--;stack[++top]=p[i];}}void rotating_colipers(){int ans=0,x=1; stack[top+1]=p[0];for(int i=0;i<=top;i++){while(get_cross(stack[x+1],stack[i+1],stack[i])>get_cross(stack[x],stack[i+1],stack[i])){x=(x+1)%top;}ans=max(ans,get_dis(stack[x],stack[i]));ans=max(ans,get_dis(stack[x+1],stack[i+1]));}printf("%d",ans);}void work(){if(N==2){printf("%d",get_dis(p[0],p[1]));}else{find_point();sort(p+1,p+N,cmp);Graham();rotating_colipers();}}void input(){N=read();for(int i=0;i<N;i++){p[i].x=read(); p[i].y=read();}}int main(){input();work();return 0;}

开始我在poj上wa了20多次之后，我作死的在百度知道上提了一个问题，然后有一位神犇迅速的帮我解决了问题。他在我的代码上进行了一点点的修改，还删去了我的读入优化，但是跑的比我加了读入优化的快一倍多Orz。

贴上他帮我修改的代码。

#include<cstdio>#include<cstring>#include<iostream>#include<algorithm>using namespace std; const int maxn=100000+10; int N,top; struct node{    int x,y;}p[maxn],point,stack[maxn]; int get_cross(node a,node b,node c){    return (b.x-a.x)*(c.y-a.y)-(b.y-a.y)*(c.x-a.x);} int get_dis(node a,node b){    return (a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y);} void find_point(){    int col=0; point=p[0];    for(int i=1;i<N;i++){        if(p[i].y<point.y || (p[i].y==point.y && p[i].x<point.x)){            col=i; point=p[i];        }    }    swap(p[0],p[col]);} bool cmp(node a,node b){//we sort these points as the clockwise.    int t=get_cross(point,a,b);    if(t<0)return true;    if(t>0)return false;    int xx=get_dis(point,a),yy=get_dis(point,b);    return xx<yy;} void Graham(){    top=1; p[N]=p[0];    stack[0]=p[0]; stack[1]=p[1];    for(int i=2;i<N;i++){//it rotates as the clockwise.        while(top>=1 && get_cross(stack[top-1],stack[top],p[i])>=0)top--;        stack[++top]=p[i];    }}void rotating_colipers(){    int ans=0,x=2; stack[top+1]=stack[0];    for(int i=1;i<=top;i++){        while(get_cross(stack[x+1],stack[i+1],stack[i])>get_cross(stack[x],stack[i+1],stack[i])){            x=(x+1)%(top+1);        }        ans=max(ans,get_dis(stack[x],stack[i]));        ans=max(ans,get_dis(stack[x],stack[i+1]));        ans=max(ans,get_dis(stack[x+1],stack[i+1]));        ans=max(ans,get_dis(stack[x+1],stack[i]));    }    printf("%d",ans);}int main(){    scanf("%d", &N);    for(int i=0;i<N;i++){        scanf("%d%d",&p[i].x, &p[i].y);    }    if(N==2)printf("%d",get_dis(p[0],p[1]));    else{        find_point();        sort(p+1,p+N,cmp);        Graham();        rotating_colipers();    }    puts("");    return 0;}

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