第一道凸包
采用Andrew算法，不论实现还是理解都非常简单
1 var q,x,y:array[0..5010] of longint; 2 i,j,k,m,n:longint; 3 ans:double; 4 5 procedure swap(var a,b:longint); 6 var c:longint; 7 begin 8 c:=a; 9 a:=b; 10 b:=c; 11 end; 12 13 procedure sort(l,r: longint); 14 var i,j,p,q: longint; 15 begin 16 i:=l; 17 j:=r; 18 p:=x[(l+r) shr 1]; 19 q:=y[(l+r) shr 1]; 20 repeat 21 while (x[i]<p) or (x[i]=p) and (y[i]<q) do inc(i); 22 while (p<x[j]) or (p=x[j]) and (q<y[j]) do dec(j); 23 if not(i>j) then 24 begin 25 swap(x[i],x[j]); 26 swap(y[i],y[j]); 27 inc(i); 28 j:=j-1; 29 end; 30 until i>j; 31 if l<j then sort(l,j); 32 if i<r then sort(i,r); 33 end; 34 35 function check(i,j,k:longint):longint; 36 begin 37 exit((y[i]-y[k])*(x[j]-x[k])-(y[j]-y[k])*(x[i]-x[k])); 38 end; 39 40 function calc(i,j:longint):double; 41 begin 42 exit(sqrt(sqr(x[i]-x[j])+sqr(y[i]-y[j]))); 43 end; 44 45 begin 46 readln(n); 47 for i:=1 to n do 48 readln(x[i],y[i]); 49 sort(1,n); 50 m:=1; 51 q[1]:=1; 52 for i:=2 to n do 53 begin 54 while (m>1) and (check(i,q[m],q[m-1])<0) do dec(m); 55 inc(m); 56 q[m]:=i; 57 end; 58 k:=m-1; 59 for i:=n-1 downto 1 do 60 begin 61 while (m>k) and (check(i,q[m],q[m-1])<0) do dec(m); 62 inc(m); 63 q[m]:=i; 64 end; 65 for i:=2 to m do 66 ans:=ans+calc(q[i],q[i-1]); 67 writeln(ans:0:2); 68 end.
