F 乘法（第k大问题）（二分）

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
#define pb push_back
#define lson root<<1,l,midd
#define rson root<<1|1,midd+1,r
const int M=1e6+6;
const ll inf=1e13;
ll a[M],b[M];
int n,m;
ll k; 
ll check(ll x){
    ll countt=0;
    for(int i=1;i<=n;i++){///在b数组里找哪些数乘当前a[i]会大于等于x,记录这个数有几个
        ll y;
        if(a[i]<0){
            if(x<=0)
                y=x/a[i];
            else
                y=floor((double)x/(double)a[i]);
            countt+=upper_bound(b+1,b+1+m,y)-b-1;///要把等于的算进去 
        }
        else if(a[i]==0&&x<=0)
            countt+=m;
        else if(a[i]>0){
            if(x<=0)
                y=x/a[i];
            else
                y=ceil((double)x/(double)a[i]);
            countt+=m-(lower_bound(b+1,b+1+m,y)-b-1);
        }
    }
    return countt;
}
int main(){
    
    scanf("%d%d%lld",&n,&m,&k);
    for(int i=1;i<=n;i++)
        scanf("%lld",&a[i]);
    for(int i=1;i<=m;i++)
        scanf("%lld",&b[i]);
    
    sort(b+1,b+1+m);
    ll l=-inf,r=inf;
    while(l<=r){
        ll midd=(l+r)>>1;
        if(check(midd)<k)
            r=midd-1;
        else
            l=midd+1;
    }
    printf("%lld\n",r);
    return 0;
}