51nod 1105 第K大的数

#include<algorithm>
#include<iostream>
#include<cstdio>
#include<cstring>
#define ll long long
#define maxn 50005
using namespace std;
ll n,k,ai[maxn],bi[maxn];

bool check(ll now)
{
    ll num=0;
    for(ll i=1;i<=n;i++)
    {
        ll l=1,r=n,mid,ans=0;
        while(l<=r)
        {
            mid=(l+r)>>1;
            if(ai[i]*bi[mid]<now) ans=mid,l=mid+1;
            else r=mid-1;
        }
        num+=ans;
    }
    if(num<k) return true;
    else return false;
}

int main()
{
    scanf("%lld%lld",&n,&k); k=(n*n)-k+1;
    for(ll i=1;i<=n;i++)    cin>>ai[i]>>bi[i];
    sort(ai+1,ai+n+1);
    sort(bi+1,bi+n+1);
    ll l=ai[1]*bi[1];
    ll r=ai[n]*bi[n];
    ll mid,ans=0;
    while(l<=r)
    {
        mid=(l+r)>>1;
        if(check(mid)) ans=mid,l=mid+1;
        else r=mid-1;
    }
    cout<<ans;
    return 0;
}