Poj 2388 Who's in the Middle 寻找第k大数

FJ is surveying his herd to find the most average cow. He wants to know how much milk this 'median' cow gives: half of the cows give as much or more than the median; half give as much or less. 

Given an odd number of cows N (1 <= N < 10,000) and their milk output (1..1,000,000), find the median amount of milk given such that at least half the cows give the same amount of milk or more and at least half give the same or less.

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显然题目简化后，就是求寻找第k大数。qsort改动一下即可。

var  a:array[1..1000000] of longint;  i,n:longint;function divi(l,r:longint):longint;var    mid,x,i,j:longint;  begin  randomize;  mid:=random(r-l+1)+l;  x:=a[mid];a[mid]:=a[l];    i:=l;j:=r;  while i<j do begin    while (i<j) and (a[j]>=x) do dec(j);if i<j then begin a[i]:=a[j];inc(i);end;    while (i<j) and (a[i]<=x) do inc(i);if i<j then begin a[j]:=a[i];dec(j);end;    end;  if i=n div 2 then exit(x);  if n div 2<i then exit(divi(l,mid-1)) else exit(divi(mid+1,r));end;begin  read(n);  for i:=1 to n do read(a[i]);  writeln(divi(1,n));end.