bzoj4232: [Neerc2011 Northern]Kids Like Cakes

#include<bits/stdc++.h>
typedef long long i64;
struct pos{int x,y;}ps[10007];
pos operator+(pos a,pos b){return (pos){a.x+b.x,a.y+b.y};}
pos operator-(pos a,pos b){return (pos){a.x-b.x,a.y-b.y};}
i64 operator*(pos a,pos b){return i64(a.x)*b.y-i64(a.y)*b.x;}
int n,ws[5007];
i64 ans=0,s0[5007],ms[5007];
void maxs(i64&a,i64 b){if(a<b)a=b;}
int main(){
    scanf("%d",&n);
    for(int i=1;i<=n;++i)scanf("%d%d",&ps[i].x,&ps[i].y);
    if((ps[2]-ps[1])*(ps[3]-ps[1])<0)std::reverse(ps+1,ps+n+1);
    ps[n*2+1]=ps[1];
    ps[0]=ps[n];
    for(int i=1;i<=n;++i)ps[n+i]=ps[i],ws[i]=i;
    for(int d=3;d<n;++d){
        for(int l=1,r=d;l<=n;++l,++r){
            pos a=ps[r]-ps[l];
            s0[l]=(ps[ws[l]]-ps[l])*a;
            while(ws[l]+1<r){
                i64 s1=(ps[ws[l]+1]-ps[l])*a;
                if(s1<s0[l])break;
                ++ws[l];
                s0[l]=s1;
            }
            maxs(ms[l],s0[l]);
        }
        ms[n+1]=ms[1];
        for(int l=1;l<=n;++l)maxs(ms[l],ms[l+1]);
    }
    for(int l=1,r=n-1;l<=n;++l,++r)maxs(ans,ms[l]-(ps[r]-ps[l])*(ps[r+1]-ps[l]));
    printf("%lld.%lld\n",ans/2,ans%2*5);
    return 0;
}