计算几何 _ 凸包

代码

#include<bits/stdc++.h>
#define int long long
#define pdd pair<double ,double >

using namespace std;
const int N=1e5+10;
const int mod=1000000007;
int n;
pdd q[N];
bool used[N];
int stk[N];
pdd operator-(pdd a,pdd b){
  return {a.first-b.first,a.second-b.second};
}
double cross(pdd a,pdd b){
  return a.first*b.second-a.second*b.first;
}
double area(pdd a,pdd b,pdd c){
  return cross(b-a,c-a);
}
double get_dist(pdd a,pdd b){
   double dx=a.first-b.first;
   double dy=a.second-b.second;
   return sqrt(dx*dx+dy*dy);
}
double  andrew(){
  sort(q,q+n);
  int top=0;
  for(int i=0;i<n;i++){
    while(top>=2 && area(q[stk[top-1]],q[stk[top]],q[i])<=0){
       // 凸包边界上的点即使被从栈中删掉，也不能删掉used上的标记
        if(area(q[stk[top-1]],q[stk[top]],q[i])<0)
          used[stk[top--]]=false;
        else top--;
      }
    stk[++top]=i;
    used[i]=true;
  }
  used[0]=false;
  for(int i=n-1;i>=0;i--){
    if(used[i])continue;
    while(top>=2 && area(q[stk[top-1]],q[stk[top]],q[i])<=0) 
      top--;
    stk[++top]=i;
  }
  double ans=0;
  for(int i=2;i<=top;i++){
    ans+= get_dist(q[stk[i-1]],q[stk[i]]);
  }
  return ans;
}
signed main(){
  cin>>n;
  for(int i=0;i<n;i++){
    cin>>q[i].first >> q[i].second;
  }
double ans=andrew();
   printf("%.2lf\n", ans);
  
    return 0;
}

标签：int,double,top,凸包,second,计算,几何,pdd,first
来源： https://www.cnblogs.com/kingwz/p/16676327.html