HDU 1392 Surround the Trees

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

典型的凸包问题

#include <stdio.h>#include <stdlib.h>#include <math.h>#define _DEBUG 1#define MAX_TREE 110typedef struct {double x,y;}Point;Point pts[MAX_TREE];Point chs[MAX_TREE];inline void exchange(Point &pa,Point &pb){Point tmp = pa;pa = pb;pb = tmp;}double dist(Point pa,Point pb){return sqrt((pa.x-pb.x)*(pa.x-pb.x)+(pa.y-pb.y)*(pa.y-pb.y));}double crossMultiple(Point p0,Point p1,Point p2){return (p1.x-p0.x)*(p2.y-p0.y)-(p2.x-p0.x)*(p1.y-p0.y);}int compare(const void *p1,const void *p2){Point p3 = *(Point *)p1;Point p4 = *(Point *)p2;double cm = crossMultiple(pts[0],p3,p4);if(cm < 0)return 1;else if(cm == 0 && dist(pts[0],p3) > dist(pts[0],p4))return 1;elsereturn -1;}double convexHull(int n){int i;int tmp=0;double sum = 0;for(i=1;i<n;++i){//寻找第0个点if(pts[i].y < pts[tmp].y)tmp = i;else if(fabs(pts[i].y-pts[tmp].y)<0.000001){//不严格if(pts[i].x < pts[tmp].x)tmp=i;}}exchange(pts[tmp],pts[0]);qsort(pts+1,n-1,sizeof(Point),compare);//排序,注意不包含第0个chs[0] = pts[0];chs[1] = pts[1];int top = 1;for(i=2;i<n;++i){while(top>=1 && crossMultiple(chs[top-1],chs[top],pts[i]) <= 0)top--;chs[++top] = pts[i]; }chs[++top] = chs[0];for(i=0;i<top;i++){sum += dist(chs[i],chs[i+1]);       }return sum;}int main(){int n;int i;#if _DEBUG==1freopen("hdu1392.in","r",stdin);#endifwhile(scanf("%d",&n) && n!=0){for(i=0;i<n;++i){scanf("%lf %lf",&pts[i].x,&pts[i].y);}if(n==1){printf("%.2f\n",0.00);continue;}if(n==2){//注意两个点的情况：n=2的时候，输出那2个点的EuclidDisprintf("%.2lf\n",dist(pts[0],pts[1]));continue;}double sum = convexHull(n);printf("%.2lf\n",sum);}return 0;}