Luogu P1094 纪念品分组

 1 var
 2   n,w,i,j,ans:Longint;
 3   a:array[1..30000] of longint;
 4   b:array[1..30000] of Boolean;
 5 
 6 procedure qsort(l,r: longint);
 7 var
 8    i,j,x: longint;
 9    y:longint;
10 begin
11     i:=l;   j:=r;
12     x:=a[(l+r) div 2];
13    repeat
14      while (a[i]>x)  do inc(i);
15      while  (x>a[j]) do dec(j);
16      if not(i>j) then
17        begin
18          y:=a[i];  a[i]:=a[j];   a[j]:=y;
19          inc(i);   j:=j-1;
20        end;
21    until i>j;
22    if l<j then qsort(l,j);
23    if i<r then qsort(i,r);
24 end;
25 
26 begin
27    // assign(input,‘t.in‘);  assign(output,‘t.out‘);
28     reset(input);  rewrite(output);
29 
30     readln(w);
31     readln(n);
32     for i:= 1 to n do readln(a[i]);
33 
34     qsort(1,n);
35 
36     i:=1;  j:=n;
37     while i<=j do
38     begin
39         if a[i]+a[j]<=w then dec(j) ;
40         inc(i);
41         ans:=ans+1;
42     end;
43 
44     writeln(ans);
45     close(input); close(output);
46 end.