51nod 1105 第K大的数

#include<cstdio>
#include<cstring>
#include<algorithm>
using std::sort;
const int N=50010;
#define ll long long 
ll a[N],b[N];
int n,k;
inline ll check(ll x)
{
    int sum=0;
    for(int i=1;i<=n;i++)
    {
//        printf("!!!\n");
        int l=1,r=n;
        ll ans,mid;
        while(l<r)
        {
            mid=(l+r)>>1;
//            printf("std:: 2 %d %d %lld\n",l,r,mid);
            ans=a[i]*b[mid];
            if(ans>x) r=mid;
            else l=mid+1;
        }
        ans=a[i]*b[l];
        if(ans>x) sum+=(n-l+1);
        else 
        {
            ans=a[i]*b[r];
            if(ans>x) sum+=(n-r+1);
        }
    }
    return sum;
}
 
int main()
{
    scanf("%d %d",&n,&k);
    for(int i=1;i<=n;i++) scanf("%lld %lld",&a[i],&b[i]);
    sort(a+1,a+1+n),sort(b+1,b+1+n);
    ll l=a[1]*b[1],r=a[n]*b[n];
    ll ans=0,mid;0
    while(r>l)
    {
        mid=(l+r)>>1;
//        printf("std 1:: %lld****\n",mid);
        ans=check(mid);
        if(ans>k-1) l=mid+1;
        else r=mid;
    }
    if(check(l)>k-1) printf("%lld\n",l);
    else printf("%lld\n",r);
    return 0;
}