HDU 1392 Surround the Trees(凸包)

来源：互联网发布：solus linux 编辑：程序博客网时间：2024/06/05 08:53

【题意】给n个点求凸包。

【分析】求凸模板，这里注意特判n==2的情况，没注意到WA一回！

【AC代码】

#include <set>#include <map>#include <math.h>#include <vector>#include <stdio.h>#include <iostream>#include <algorithm>using namespace std;const int maxn = 10010;const double PI = acos(-1);struct Point{    double x,y;    Point(){}    Point(int x,int y):x(x),y(y){}    bool operator<(const Point &rhs)const{        if(x==rhs.x) return y<rhs.y;        return x<rhs.x;    }}q[maxn],p[maxn];double Cross(const Point &a,const Point &b){    return a.x*b.y-a.y*b.x;}double get_Dis(const Point &a,const Point &b){    return (double)sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));}int main(){    int n;    while(scanf("%d",&n)!=EOF){        if(n==0) return 0;        for(int i=0; i<n; i++) scanf("%lf%lf",&q[i].x,&q[i].y);        sort(q,q+n);        if(n==1){printf("0.00\n");continue;}if(n==2){printf("%.2lf\n",get_Dis(q[0],q[1]));continue;}        int m = 0;        for(int i=0; i<n; i++){//"下凸包"            while(m>1&&Cross(Point(p[m-1].x-p[m-2].x,p[m-1].y-p[m-2].y),Point(q[i].x-p[m-2].x,q[i].y-p[m-2].y))<=0) m--;            p[m++] = q[i];        }        int k = m;        for(int i=n-2; i>=0; i--){//"上凸包"            while(m>k&&Cross(Point(p[m-1].x-p[m-2].x,p[m-1].y-p[m-2].y),Point(q[i].x-p[m-2].x,q[i].y-p[m-2].y))<=0) m--;            p[m++] = q[i];        }        if(n>1) m--;        double sum = 0;        for(int i=0,j=1; i<m; i++,j++){            sum += get_Dis(p[i],p[j]);        }        printf("%.2f\n",sum);    }    return 0;}

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