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[0..50010] 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 1];
    q:=y[(l+r) shr 1];
    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-1;
      end;
    until i>j;
    if l<j then sort(l,j);
    if i<r then sort(i,r);
  end;

begin
  readln(n);
  while n<>-1 do
  begin
    for i:=1 to n do
      readln(x[i],y[i]);
    sort(1,n);
    q[1]:=1;
    t:=1;
    for i:=2 to n do
    begin
      while (t>1) and (cross(q[t],i,q[t-1])<=0) do dec(t);
      inc(t);
      q[t]:=i;
    end;
    k:=t;
    for i:=n-1 downto 1 do
    begin
      while (t>k) and (cross(q[t],i,q[t-1])<=0) do dec(t);
      inc(t);
      q[t]:=i;
    end;
    for i:=2 to t-1 do
      q[t+i-1]:=q[i];
    ans:=cross(q[3],q[2],q[1]);
    j:=2;
    k:=3;
    for i:=1 to t-3 do
    begin
      k:=max(k,i+2);
      j:=max(j,i+1);
      while (k<i+t-2) and (cross(q[j],q[k+1],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+1],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/2:0:2);
    readln(n);
  end;
end.