zoj 1453 Surround the Trees Andrew算法

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#include <stdio.h>#include <string.h>#include <iostream>#include <algorithm>#include <vector>#include <queue>#include <stack>#include <set>#include <map>#include <string>#include <math.h>#include <stdlib.h>#include <time.h>using namespace std;struct Point{    int x,y;}p[1024],ch[1024];Point operator - (Point a,Point b){    Point c;    c.x=a.x-b.x;    c.y=a.y-b.y;    return c;}int cross(Point a,Point b){    return a.x*b.y-b.x*a.y;}double dot(Point a,Point b){    return sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));}bool cmp(Point a,Point b){    if(a.x==b.x) return a.y<b.y;    else return a.x<b.x;}int main(){    int i,n,m;    double ans;    while(~scanf("%d",&n))    {        if(n==0) break;        for(i=0;i<n;i++)            scanf("%d%d",&p[i].x,&p[i].y);        if(n==1){printf("0.00\n");continue;}        else if(n==2) {printf("%.2f\n",dot(p[0],p[1])*2);continue;}        sort(p,p+n,cmp);        m=0;        for(i=0;i<n;i++)        {            while(m>1&&cross(ch[m-1]-ch[m-2],p[i]-ch[m-2])<=0) m--;            ch[m++]=p[i];        }        int k=m;        for(i=n-2;i>=0;i--)        {            while(m>k&&cross(ch[m-1]-ch[m-2],p[i]-ch[m-2])<=0) m--;            ch[m++]=p[i];        }        if(n>1) m--;        ans=0;        for(i=0;i<m;i++)        {            ans+=dot(ch[i],ch[i+1]);        }        printf("%.2f\n",ans);    }    return 0;}
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