忘记附上代码了。。。
代码实现：
type hh=record
x,y:longint;
end;
var a:array[1..100001] of hh;
i,n:longint;
zhan,zhan1:array[1..50001] of longint;
pp,p:longint;
x1,y1,x2,y2,temp:longint;
ans:double;
procedure sort1(l,r: longint);
var
i,j,x: longint;
y:hh;
begin
i:=l;
j:=r;
x:=a[(l+r) div 2].x;
repeat
while a[i].x<x do
inc(i);
while x<a[j].x do
dec(j);
if not(i>j) then
begin
y:=a[i];
a[i]:=a[j];
a[j]:=y;
inc(i);
j:=j-1;
end;
until i>j;
if l<j then
sort1(l,j);
if i<r then
sort1(i,r);
end;
procedure sort2(l,r: longint);
var
i,j,x: longint;
y:hh;
begin
i:=l;
j:=r;
x:=a[(l+r) div 2].y;
repeat
while a[i].y<x do
inc(i);
while x<a[j].y do
dec(j);
if not(i>j) then
begin
y:=a[i];
a[i]:=a[j];
a[j]:=y;
inc(i);
j:=j-1;
end;
until i>j;
if l<j then
sort2(l,j);
if i<r then
sort2(i,r);
end;
begin
readln(n);
for i:= 1 to n do
readln(a[i].x,a[i].y);
sort1(1,n);
for i:= 1 to n do
begin
if a[i].x<>a[i+1].x then if temp=0 then continue;
if a[i].x=a[i+1].x then if temp<>0 then continue;
if a[i].x=a[i+1].x then if temp=0 then temp:=i;
if a[i].x<>a[i+1].x then if temp<>0 then begin sort2(temp,i);temp:=0;end;
end;
inc(pp);
zhan[pp]:=1;
inc(pp);
zhan[pp]:=2;
for i:= 2 to n-1 do
begin
x1:=a[zhan[pp]].x-a[zhan[pp-1]].x;
y1:=a[zhan[pp]].y-a[zhan[pp-1]].y;
x2:=a[i+1].x-a[zhan[pp]].x;
y2:=a[i+1].y-a[zhan[pp]].y;
if x1*y2-x2*y1<0 then
begin
inc(pp);
zhan[pp]:=i+1;
end
else
begin
repeat
dec(pp);
x1:=a[zhan[pp]].x-a[zhan[pp-1]].x;
y1:=a[zhan[pp]].y-a[zhan[pp-1]].y;
x2:=a[i+1].x-a[zhan[pp]].x;
y2:=a[i+1].y-a[zhan[pp]].y;
until (x1*y2-x2*y1<0) or (pp=1);
inc(pp);
zhan[pp]:=i+1;
end;
end;
inc(p);
zhan1[p]:=n;
inc(p);
zhan1[p]:=n-1;
for i:= n-1 downto 2 do
begin
x1:=a[zhan1[p]].x-a[zhan1[p-1]].x;
y1:=a[zhan1[p]].y-a[zhan1[p-1]].y;
x2:=a[i-1].x-a[zhan1[p]].x;
y2:=a[i-1].y-a[zhan1[p]].y;
if x1*y2-x2*y1<0 then
begin
inc(p);
zhan1[p]:=i-1;
end
else
begin
repeat
dec(p);
x1:=a[zhan1[p]].x-a[zhan1[p-1]].x;
y1:=a[zhan1[p]].y-a[zhan1[p-1]].y;
x2:=a[i-1].x-a[zhan1[p]].x;
y2:=a[i-1].y-a[zhan1[p]].y;
until (x1*y2-x2*y1<0) or (p=1);
inc(p);
zhan1[p]:=i-1;
end;
end;
for i:= 1 to pp-1 do
ans:=ans+sqrt((a[zhan[i+1]].x-a[zhan[i]].x)*(a[zhan[i+1]].x-a[zhan[i]].x)+(a[zhan[i+1]].y-a[zhan[i]].y)*(a[zhan[i+1]].y-a[zhan[i]].y));
for i:= 1 to p-1 do
ans:=ans+sqrt((a[zhan1[i+1]].x-a[zhan1[i]].x)*(a[zhan1[i+1]].x-a[zhan1[i]].x)+(a[zhan1[i+1]].y-a[zhan1[i]].y)*(a[zhan1[i+1]].y-a[zhan1[i]].y));
writeln(ans:0:1);
end.