HDU 1392 Surround the Trees（求凸包周长）

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <climits>
#include <cmath>
using namespace std;
const int MAX_N=1100;

int n;

struct Point{
double x,y;
Point () {}
Point (double x,double y) : x(x),y(y){
}
Point operator + (const Point& rhs) const {
return Point(x+rhs.x,y+rhs.y);
}
Point operator - (const Point& rhs) const {
return Point(x-rhs.x,y-rhs.y);
}
Point operator *(const double d) const {
return Point(d*x,d*y);
}
double dis(const Point& rhs) const {
return sqrt((x-rhs.x)*(x-rhs.x)+(y-rhs.y)*(y-rhs.y));
}
double cross(const Point& rhs) const {
return (x*rhs.y-y*rhs.x);
}
}point[MAX_N],vertex[MAX_N];

bool cmp_x(const Point& a,const Point& b)
{
if(a.x==b.x) return a.y<b.y;
else return a.x<b.x;
}

int Andrew()
{
sort(point,point+n,cmp_x);
int k=0;
for(int i=0;i<n;i++){
while(k>1&&(vertex[k-1]-vertex[k-2]).cross(point[i]-vertex[k-1])<=0) k--;
vertex[k++]=point[i];
}
int m=k;
for(int i=n-2;i>=0;i--){
while(k>m&&(vertex[k-1]-vertex[k-2]).cross(point[i]-vertex[k-1])<=0) k--;
vertex[k++]=point[i];
}
if(k>1) k--;
return k;
}

void solve()
{
int total=Andrew();
//printf("total=%d\n",total);
double perimeter=0;
for(int i=0;i<total;i++){
perimeter+=vertex[i].dis(vertex[(i+1)%total]);
}
printf("%.2f\n",perimeter);
}

int main()
{
freopen("Gin.txt","r",stdin);
while(~scanf("%d",&n)&&n){
for(int i=0;i<n;i++){
scanf("%lf%lf",&point[i].x,&point[i].y);
}
if(n==1){
printf("0\n");
}else if(n==2){
printf("%.2f\n",point[0].dis(point[1]));
}else {
solve();
}
}
return 0;
}