poj2079

根据凸包的单峰性质，穷举第一个顶点
然后先更新第三个顶点，再更新第二个顶点

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

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

 function max(a,b:longint):longint;
   begin
     if a>b then exit(a) else exit(b);
   end;
 procedure swap(var a,b:longint);
   var c:longint;
   begin
     c:=a;
     a:=b;
     b:=c;
   end;

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