找第k大的数（解法3：快速排序）

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 1 var i,k,n:longint;

 2     a:array[1..100000]of longint;

 3 

 4 procedure qsort(l,r:longint);

 5 var i,j,m,t:longint;

 6 begin

 7   if l=r then exit;

 8   i:=l; j:=r; m:=a[k];

 9   repeat

10     while a[i]<m do inc(i);

11     while a[j]>m do dec(j);

12     if i<j then

13     begin

14       t:=a[i]; a[i]:=a[j]; a[j]:=t;

15       inc(i); dec(j);

16     end;

17   until i>=j;

18   if i=k then exit;

19   if i>k then qsort(l,j) else qsort(j+1,r);

20 end;

21 

22 begin

23   assign(input,'maxk.in'); reset(input);

24   assign(output,'maxk.out'); rewrite(output);

25   read(n,k);

26   for i:=1 to n do read(a[i]);

27   qsort(1,n);

28   writeln(a[k]);

29   close(input); close(output);

30 end.