入住CSDN的第一篇文章，纪念我的第一道凸包题

Program ConvexHull_FeedingGrils;
Type point=record
x,y:real;
end;
Var s:array[0..100000] of point;
c:array[0..10000] of point;
p:point;
i,j,k,m,n,t:longint;
len:real;
Function Calc(p1,p2,p0:point):real;
begin
calc:=(p1.x-p0.x)*(p2.y-p0.y)-(p2.x-p0.x)*(p1.y-p0.y);
end;
Function Dist(p1,p0:point):real;
begin
dist:=sqrt(sqr(p1.x-p0.x)+sqr(p1.y-p0.y));
end;
Procedure Qsort(t,s:longint);
var i,j:longint;
p,mid:point;
begin
i:=t;j:=s;mid:=c[(i+j) shr 1];
repeat
while (calc(c[i],mid,c[0])>0)or(calc(c[i],mid,c[0])=0)
and(dist(c[i],c[0])<dist(mid,c[0])) do inc(i);
while (calc(c[j],mid,c[0])<0)or(calc(c[j],mid,c[0])=0)
and(dist(c[j],c[0])<dist(mid,c[0])) do dec(j);
if i<=j then
begin
p:=c[i];
c[i]:=c[j];
c[j]:=p;
inc(i);dec(j);
end;
until i>j;
if i<s then Qsort(i,s);
if t<j then Qsort(t,j);
end;
Begin
readln(n);
c[0].x:=maxlongint;c[0].y:=maxlongint;
for i:=0 to n-1 do
begin
readln(c[i].x,c[i].y);
if (c[i].y<c[0].y) or (c[i].y=c[0].y) and (c[i].x<c[0].x) then
begin
p:=c[0];
c[0]:=c[i];
c[i]:=p;
end;
end;
if n>2 then
Qsort(1,n-1);
for i:=1 to 3 do
begin
if (c[i].x=0 )and(c[i].y=0) then break; //要加这一句，否则会WA一个点 106错误
s[i]:=c[i-1];
t:=i;
end;
for i:=3 to n-1 do
begin
while (t<>0)and(calc(s[t],c[i],s[t-1])<=0) do
dec(t);
inc(t);
s[t]:=c[i];
end;
len:=0;
for i:=2 to t do
len:=len+dist(s[i],s[i-1]);
if t<>0 then
len:=len+dist(s[t],s[1]);
writeln(len:0:2);
End.