出自：https://blog.csdn.net/queque_heiya/article/details/106185459
完整代码如下：
#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
#define ll long long
const int maxa=1e7+10;
using namespace std;
const int N=1e7+10;
int T,n,k,aa[N];
struct FastIO{FILE *fp;static const int S=1e7;typedef long long ll;int wpos;char wbuf[S];FastIO():wpos(0){}inline int xchar(){static char buf[S];static int len=0,pos=0;if(pos==len)pos=0,len=fread(buf,1,S,stdin);return buf[pos++];}inline int xuint(){int c=xchar(),x=0;while(c<=32)c=xchar();for(;'0'<=c&&c<='9';c=xchar())x=x*10+c-'0';return x;}inline int xint(){int s=1,c=xchar(),x=0;while(c<=32)c=xchar();if(c=='-')s=-1,c=xchar();for(;'0'<=c&&c<='9';c=xchar())x=x*10+c-'0';return x*s;}inline void xstring(char *s){int c=xchar();while(c<=32)c=xchar();for(;c>32;c=xchar())*s++=c;*s=0;}inline void wchar(int x){if(wpos==S)fwrite(wbuf,1,S,stdout);wbuf[wpos++]=x;}inline void wint(ll x){if(x<0)wchar('-'),x=-x;char s[24];int n=0;while(x||!n)s[n++]='0'+x%10,x/=10;while(n--)wchar(s[n]);wchar('\n');}inline void wstring(const char *s){while(*s)wchar(*s++);}~FastIO(){if(wpos)fwrite(wbuf,1,wpos,stdout),wpos=0;}
}io;
void sort(int l, int k, int r){if(l>=r) return;swap(aa[l],aa[(l+r)>>1]);int L=l,R=r-1,x=aa[l];while(L<R){while(L<R&&aa[R]>=x)R--;aa[L]=aa[R];while(L<R&&aa[L]<=x)L++;aa[R]=aa[L];}aa[L]=x;int left=L-l+1;if(left>k)sort(l,k,L);if(k>left)sort(L+1,k-left,r);
}
int main(){int T;T=io.xint();while(T--){n=io.xint(),k=io.xint();for(int i=0;i<n;++i)aa[i]=io.xint();sort(0,n+1-k,n);//nth_element(aa,aa+n-k,aa+n);io.wint(aa[n-k]);}return 0;
}