论快排的正确性……
proc...... qsort(l,r:longint);
var i,j,p,mid:longint;
begin
i:=l;
j:=r;
mid:=a[(i+j) div 2];
repeat
while a[i]<mid do inc(i);
while a[j]>mid do dec(j);
if i<=j then begin
p:=a[i];
a[i]:=a[j];
a[j]:=p;
inc(i);
dec(j);
end;
until i>j;
if i<r then qsort(i,r);
if l<j then qsort(l,j);
end;
