BZOJ1670 [Usaco2006 Oct]Building the Moat护城河的挖掘

裸的凸包。。。（和旋转卡壳有什么关系吗。。。蒟蒻求教T T）

话说忘了怎么写了。。。（我以前都是先做上凸壳再做下凸壳的说）

于是看了下hzwer的写法，用了向量的点积，方便多了，于是果断学习(Orz)了！

竟然比原作者要快T T

 /**************************************************************
     Problem: 1670
     User: rausen
     Language: C++
     Result: Accepted
     Time:16 ms
     Memory:900 kb
 ****************************************************************/
 
 #include <cstdio>
 #include <cmath>
 #include <algorithm>
 
 #define points P
 using namespace std;
 typedef long long ll;
 const int N = ;
 int n, top;
 double ans;
 struct points{
     int x, y;
 }p[N], s[N];
 
 inline ll operator * (const P a, const P b){
     return a.x * b.y - a.y * b.x;
 }
 
 inline P operator - (const P a, const P b){
     P tmp;
     tmp.x = a.x - b.x, tmp.y = a.y - b.y;
     return tmp;
 }
 
 inline ll Sqr(const ll x){
     return x * x;
 }
 
 inline ll dis(const P a, const P b){
     return Sqr(a.x - b.x) + Sqr(a.y - b.y);
 }
 
 inline bool operator < (const P a, const P b){
     ll tmp = (a - p[]) * (b - p[]);
     return tmp ==  ? dis(p[], a) < dis(p[], b) : tmp > ;
 }
 
 inline int read(){
     int x = , sgn = ;
     char ch = getchar();
     while (ch < '' || ch > ''){
         if (ch == '-') sgn = -;
         ch = getchar();
     }
     while (ch >= '' && ch <= ''){
         x = x *  + ch - '';
         ch = getchar();
     }
     return sgn * x;
 }
 
 int work(){
     int k = ;
     for (int i = ; i <= n; ++i)
         if (p[i].y < p[k].y || (p[i].y == p[k].y && p[i].x < p[k].x)) k = i;
     swap(p[], p[k]);
     sort(p + , p + n + );
     top = , s[] = p[], s[] = p[];
     for (int i = ; i <= n; ++i){
         while ((s[top] - s[top - ]) * (p[i] - s[top]) <= ) --top;
         s[++top] = p[i];
     }
     s[top + ] =  p[];
     for (int i = ; i <= top; ++i)
         ans += sqrt(dis(s[i], s[i + ]));
 }
 
 int main(){
     n = read();
     for (int i = ; i <= n; ++i)
         p[i].x = read(), p[i].y = read();
     work();
     printf("%.2lf\n", ans);
 }