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== 难得的y出了一道计算几何。。
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <math.h>
#include <iostream>
using namespace std;
#define INF 999999999.9
#define PI acos(-1.0)
#define ll long long
struct Point
{
ll x, y, dis;
}pt[400005], stack[400005], p0;
ll top, tot;
//计算几何距离
ll Dis(ll x1, ll y1, ll x2, ll y2)
{
return (x1-x2)*(x1-x2)+(y1-y2)*(y1-y2);
}
//极角比较， 返回-1: p0p1在 p0p2的右侧，返回 0:p0,p1,p2共线
int Cmp_PolarAngel(struct Point p1, struct Point p2, struct Point pb)
{
double delta=(p1.x-pb.x)*(p2.y-pb.y)-(p2.x-pb.x)*(p1.y-pb.y);
if (delta<0.0) return 1;
else if (delta==0.0) return 0;
else return -1;
}
// 判断向量p2p3是否对 p1p2构成左旋
bool Is_LeftTurn(struct Point p3, struct Point p2, struct Point p1)
{
int type=Cmp_PolarAngel(p3, p1, p2);
if (type<0) return true;
return false;
}
//先按极角排，再按距离由小到大排
int Cmp(const void*p1, const void*p2)
{
struct Point*a1=(struct Point*)p1;
struct Point*a2=(struct Point*)p2;
int type=Cmp_PolarAngel(*a1, *a2, p0);
if (type<0) return -1;
else if (type==0)
{
if (a1->dis<a2->dis) return -1;
else if (a1->dis==a2->dis) return 0;
else return 1;
}
else return 1;
}
//求凸包
ll step[4][2] = {0,1,0,-1,1,0,-1,0};
void Solve(ll n)
{
ll i, k;
p0.x=p0.y=INF;
ll x,y;
for (i=0;i<n;i++)
{
scanf("%I64d %I64d",&x, &y);
for(ll j = 0; j < 4; j++)
{
Point P = {x+step[j][0], y+step[j][1]};
pt[i*4+j] = P;
if (pt[i*4+j].y < p0.y)
{
p0.y=pt[i*4+j].y;
p0.x=pt[i*4+j].x;
k=i*4+j;
}
else if (pt[i*4+j].y==p0.y)
{
if (pt[i*4+j].x<p0.x)
{
p0.x=pt[i*4+j].x;
k=i*4+j;
}
}
}
}
n *= 4;
pt[k]=pt[0];
pt[0]=p0;
for (i=1;i<n;i++)
pt[i].dis=Dis(pt[i].x,pt[i].y, p0.x,p0.y);
qsort(pt+1, n-1, sizeof(struct Point), Cmp);
//去掉极角相同的点
tot=1;
for (i=2;i<n;i++)
if (Cmp_PolarAngel(pt[i], pt[i-1], p0))
pt[tot++]=pt[i-1];
pt[tot++]=pt[n-1];
top=1;
stack[0]=pt[0];
stack[1]=pt[1];
for (i=2;i<tot;i++)
{
while (top>=1 && Is_LeftTurn(pt[i], stack[top], stack[top-1])==false)
top--;
stack[++top]=pt[i];
}
}
inline ll Abs(ll x){return x>0?x:-x;}
ll len(Point a, Point b){
return Abs(a.x-b.x) + Abs(a.y-b.y)-min(Abs(a.x-b.x), Abs(a.y-b.y));
}
int main ()
{
ll n, x, y;
while (cin>>n)
{
Solve(n);
ll ans = 0;
for(ll i = 0; i < top; i++)
ans += len(stack[i], stack[i+1]);
ans += len(stack[top], stack[0]);
cout<<ans<<endl;
}
return 0;
}
Codeforces 50C Happy Farm 5 凸包,布布扣,bubuko.com
Codeforces 50C Happy Farm 5 凸包
原文：http://blog.csdn.net/qq574857122/article/details/38171357