POJ 3348 Cows

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一开始以为是个求面积的，后来发现先要求凸包，然后求凸包面积

有模板的话很好写

#include<stdio.h>#include<string>#include<math.h>#include<iostream>#include<algorithm>using namespace std;const int MAXN=10005;const double eps = 1e-8;int sgn(double x){if(fabs(x) < eps)return 0;if(x < 0)return -1;else return 1;}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);}//叉积double operator ^(const Point &b)const{return x*b.y - y*b.x;}//点积double operator *(const Point &b)const{return x*b.x + y*b.y;}};struct Line{Point s,e;Line(){}//构造函数Line(Point _s,Point _e){s = _s;e = _e;}};double dist(Point a,Point b){return sqrt((a-b)*(a-b));}//变量申明Point list[10005];Point p[10005];int Stack[MAXN],top;double CalcArea(Point p[],int n){double res = 0;for(int i = 0;i < n;i++)res += (p[i]^p[(i+1)%n])/2;return fabs(res);}bool _cmp(Point p1,Point p2){double tmp = (p1-list[0])^(p2-list[0]);if(sgn(tmp) > 0)return true;else if(sgn(tmp) == 0 && sgn(dist(p1,list[0]) - dist(p2,list[0])) <= 0)return true;else return false;}void Graham(int n){Point p0;int k = 0,i;p0 = list[0];//找最下边的一个点for(i = 1;i < n;i++){if( (p0.y > list[i].y) || (p0.y == list[i].y && p0.x > list[i].x) ){p0 = list[i];k = i;}}swap(list[k],list[0]);sort(list+1,list+n,_cmp);if(n == 1){top = 1;Stack[0] = 0;return;}if(n == 2){top = 2;Stack[0] = 0;Stack[1] = 1;return ;}Stack[0] = 0;Stack[1] = 1;top = 2;for(i = 2;i < n;i++){while(top > 1 && sgn((list[Stack[top-1]]-list[Stack[top-2]])^(list[i]-list[Stack[top-2]])) <= 0)top--;Stack[top++] = i;}}int main() {#ifndef ONLINE_JUDGEfreopen("in.txt","r",stdin);#endifint n;scanf("%d",&n);int i;for(i=0;i<n;i++) {scanf("%lf%lf",&list[i].x,&list[i].y);}Graham(n);for(i=0;i<top;i++) {p[i].x=list[Stack[i]].x;p[i].y=list[Stack[i]].y;}int res=CalcArea(p,top)/50;printf("%d\n",res);return 0;}

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