凸包（转自水神百度文库）

#include<stdio.h>#include<math.h>#include<algorithm>using namespace std;struct Point{    double x,y,len;}Pt[20000],Stack[20000],Point_A;double Cross(Point a,Point b,Point c){    return (b.x-a.x)*(c.y-a.y)-(b.y-a.y)*(c.x-a.x);}double Dis(Point a,Point b){    return sqrt(pow(a.x-b.x,2)+pow(a.y-b.y,2));}void FindPoint(int n){    int i,tempNumber=0;    Point tempPoint;    Point_A=Pt[0];    for(i=1;i<n;i++)    {        if(Pt[i].y<Point_A.y||Pt[i].y==Point_A.y&&Pt[i].x<Point_A.x)        {            tempNumber=i;            Point_A=Pt[i];        }    }    tempPoint=Pt[0];    Pt[0]=Pt[tempNumber];    Pt[tempNumber]=tempPoint;}bool Cmp(Point a,Point b){    double k=Cross(Point_A,a,b);    if(k>0) return true;    if(k<0) return false;    a.len=Dis(Point_A,a);    b.len=Dis(Point_A,b);    return a.len>b.len;}void Graham(int n){    int i,top=2;    Pt[n]=Pt[0];    Stack[0]=Pt[0];    Stack[1]=Pt[1];    Stack[2]=Pt[2];    for(i=3;i<=n;i++)    {        while(Cross(Stack[top-1],Stack[top],Pt[i])<=0&&top>1)        top--;        Stack[++top]=Pt[i];    }}int main(void){    int i,Num;    while(scanf("%d",&Num)!=EOF)    {        for(i=0;i<Num;i++)        scanf("%lf%lf",&Pt[i].x,&Pt[i].y);        FindPoint(Num);        sort(Pt,Pt+Num,Cmp);        Graham(Num);    }    return 0;}