凸包。

http://blog.sina.com.cn/s/blog_9dff1a750101ag0l.html

const zero=1e-6; maxn=100000;
type point=record x,y:extended; end;
var p:array[1..maxn]of point;
ch:array[1..maxn]of longint;
temp,n,m,i,j,k:longint;
function sgn(x:extended):longint; inline;
begin
if abs(x)<zero then exit(0);
if x<0 then sgn:=-1 else sgn:=1;
end;
function cross(a,b,c:point):extended; inline;
begin
cross:=(b.x-a.x)*(c.y-a.y)-(b.y-a.y)*(c.x-a.x);
end;
function dist(a,b:point):extended; inline;
begin
dist:=sqr(a.x-b.x)+sqr(a.y-b.y);
end;
function cmp(a,b,c:point):boolean; inline;
var temp:longint;
begin
temp:=sgn(cross(a,b,c));
if temp<>0 then exit(temp<0); {*B}
cmp:=dist(a,b)<dist(a,c); {*A}
end;
begin
assign(input,'Hull.in'); reset(input);
assign(output,'Hull1.out'); rewrite(output);
readln(n);
for i:=1 to n do
begin
readln(p[i].x,p[i].y);
if (j=0)or(p[i].x<p[j].x)or
(sgn(p[i].x-p[j].x)=0)and(p[i].y<p[j].y)
then j:=i;
end;
temp:=j;
while true do
begin
k:=0;
inc(m); ch[m]:=j;
for i:=1 to n do
if (i<>j)and((k=0)or
cmp(p[j],p[k],p[i]))
then k:=i;
if k=temp then break;
j:=k;
end;
for i:=1 to m do
writeln(p[ch[i]].x:0:2,' ',p[ch[i]].y:0:2);
close(input); close(output);
end.