bzoj1069

四边形的问题可以转化为三角形处理
穷举对角线，然后处理上下两个三角形，旋转卡壳

 var x,y:array[..] of double;
     q:array[..] of longint;
     l,r,i,j,k,t,n:longint;
     ans:double;

 function max(a,b:double):double;
   begin
     if a>b then exit(a) else exit(b);
   end;

 function cross(i,j,k,r:longint):double;
   begin
     exit((x[i]-x[j])*(y[k]-y[r])-(x[k]-x[r])*(y[i]-y[j]));
   end;

 procedure swap(var a,b:double);
   var c:double;
   begin
     c:=a;
     a:=b;
     b:=c;
   end;

 procedure sort(l,r: longint);
   var i,j: longint;
       p,q:double;

   begin
     i:=l;
     j:=r;
     p:=x[(l+r) shr ];
     q:=y[(l+r) shr ];
     repeat
       while (x[i]<p) or (x[i]=p) and (y[i]<q) do inc(i);
       while (p<x[j]) or (p=x[j]) and (q<y[j]) do dec(j);
       if not(i>j) then
       begin
         swap(x[i],x[j]);
         swap(y[i],y[j]);
         inc(i);
         j:=j-;
       end;
     until i>j;
     if l<j then sort(l,j);
     if i<r then sort(i,r);
   end;

 begin
   readln(n);
   for i:= to n do
     readln(x[i],y[i]);
   sort(,n);
   q[]:=;
   t:=;
   for i:= to n do
   begin
     while (t>) and (cross(q[t],q[t-],i,q[t-])<=) do dec(t);
     inc(t);
     q[t]:=i;
   end;
   k:=t;
   for i:=n- downto  do
   begin
     while (t>k) and (cross(q[t],q[t-],i,q[t-])<=) do dec(t);
     inc(t);
     q[t]:=i;
   end;
   for i:= to t- do
     q[i+t-]:=q[i];
   for i:= to t- do
   begin
     l:=i+;
     r:=i+;
     while (r<i+t-) and (cross(q[i+],q[i],q[r],q[i])<cross(q[i+],q[i],q[r+],q[i])) do inc(r);
     ans:=max(ans,cross(q[i+],q[i],q[i+],q[i])+cross(q[i+],q[i],q[r],q[i]));
     for j:=i+ to i+t- do
     begin
       while (l<j-) and (cross(q[l+],q[i],q[j],q[i])>cross(q[l],q[i],q[j],q[i])) do inc(l);
       while (r<i+t-) and (cross(q[j],q[i],q[r+],q[i])>cross(q[j],q[i],q[r],q[i])) do inc(r);
       ans:=max(ans,cross(q[l],q[i],q[j],q[i])+cross(q[j],q[i],q[r],q[i]));
     end;
   end;
   ans:=ans/;
   writeln(ans::);
 end.