hdu_1392 Surround the Trees(凸包)

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http://acm.hdu.edu.cn/showproblem.php?pid=1392

分析：用了凸包模板很快就可以出来（注意特殊情况的处理，前两次就是这么WA的）

代码：

//hdu 1392#include <cstdio>#include <cstring>#include <cmath>#include <queue>#include <iostream>#include <algorithm>using namespace std;#define MAXN 110const double pi=acos(-1.0);struct Point{    double x,y;    Point(){}    Point(double _x,double _y)    {          x = _x;          y = _y;    }    Point operator - (const Point &b)const    {       return Point(x-b.x,y-b.y);    }    double operator *(const Point &b)const    {       return x*b.x+y*b.y;    }}e[MAXN],res[MAXN];int cmp(Point a,Point b){    return a.x != b.x ? a.x<b.x : a.y<b.y;}double cross(Point a,Point b,Point c)//向量积{    return (a.x-c.x)*(b.y-c.y)-(b.x-c.x)*(a.y-c.y);}double dist(Point a,Point b){    return sqrt((a-b)*(a-b));}int convex(int n)//求凸包上的点{    sort(e,e+n,cmp);    int m=0,k;    for(int i=0;i<n;i++){        while(m>1&&cross(res[m-1],e[i],res[m-2])<=0)m--;        res[m++]=e[i];    }    k=m;    for(int i=n-2;i>=0;i--){        while(m>k&&cross(res[m-1],e[i],res[m-2])<=0)m--;        res[m++]=e[i];    }    if(n>1) m--;    return m;}int main(){    freopen("in.txt","r",stdin);    int n,m;    double ans;    while(scanf("%d",&n)!=EOF){        if(n==0) break;        ans=0;        for(int i=0;i<n;i++)            scanf("%lf%lf",&e[i].x,&e[i].y);        if(n==1) {printf("0.00\n"); continue;}        if(n==2) {printf("%.2lf\n",dist(e[0],e[1]));continue;}        m=convex(n);        for(int i=1;i<m;i++)            ans+=dist(res[i],res[i-1]);        ans+=dist(res[m-1],res[0]);        printf("%.2lf\n",ans);    }    return 0;}

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