杭电 1392 Surround the Trees

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经典凸包问题！！！！

AC代码如下：

#include<iostream>#include<cstring>#include<algorithm>#include<cmath>#include<cstdio>using namespace std;struct H{    double x,y;}trees[105];bool cmp(H a,H b){    return a.x<b.x||(a.x==b.x&&a.y<b.y);}int xj(double x1,double y1,double x2,double y2){    if(x1*y2-x2*y1<=0)        return 1;    else return 0;}int main(){    int n;    int i,j,tt;    double fx,fy,sx,sy,xx,yy;    double a[105],b[105];    while(cin>>n,n)    {        tt=0;        for(i=0;i<n;i++)            cin>>trees[i].x>>trees[i].y;        sort(trees,trees+n,cmp);        for(i=0;i<n;i++)        {            while(tt>1&&xj(a[tt-1]-a[tt-2],b[tt-1]-b[tt-2],trees[i].x-a[tt-2],trees[i].y-b[tt-2]))                tt--;            a[tt]=trees[i].x;b[tt++]=trees[i].y;        }        int k=tt;        for(i=n-2;i>=0;i--)        {            while(tt>k&&xj(a[tt-1]-a[tt-2],b[tt-1]-b[tt-2],trees[i].x-a[tt-2],trees[i].y-b[tt-2]))                tt--;            a[tt]=trees[i].x;b[tt++]=trees[i].y;        }        double sum=0;        for(i=1;i<tt;i++)        {            sum+=sqrt((a[i]-a[i-1])*(a[i]-a[i-1])+(b[i]-b[i-1])*(b[i]-b[i-1]));        }        if(n==2)            printf("%.2f\n",sum/2);        else        printf("%.2f\n",sum);    }    return 0;}

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