POJ 2299(逆序对数）

水题 裸的求逆序对数

Program P2299;
const
   maxn=500100;
Var
   tt,i,j,k,n:longint;
   a,le,re:array[1..maxn] of longint;
   ans:int64;
procedure mergesort(l,r:longint);
var
   i,j,k,mid:longint;

begin
   if l=r then exit;
   mid:=(l+r) shr 1;
   mergesort(l,mid);
   mergesort(mid+1,r);
   for i:=l to mid do le[i-l+1]:=a[i];
   le[mid+2-l]:=maxlongint;
   for i:=mid+1 to r do re[i-mid]:=a[i];
   re[r+1-mid]:=maxlongint;
   i:=1;j:=1;
   for k:=l to r do
   begin
      if (le[i]<=re[j]) then
      begin
         a[k]:=le[i];
         inc(i);
      end
      else
      begin
         a[k]:=re[j];
         inc(j);
         ans:=mid-l+1-(i-1)+ans;
      end;
   end;
end;
Begin
   while not eof do
   begin
      read(n);
      if n=0 then break;
      for i:=1 to n do read(a[i]);
      ans:=0;
      mergesort(1,n);
      writeln(ans);

   end;
end.