暴力二分答案即可獲得正解，注意邊界問題

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<ll,int> pll;
const int N=1e6+10;
ll n,m,k;
ll a[N],b[N];
bool check(ll x){
    ll cnt=0;
    int i;
    for(i=1;i<=m;i++){
        if(b[i]==0)
            cnt+=(x<0)?n:0;
        if (b[i]<0
 
)
            cnt+=lower_bound(a+1, a+n+1, ceil((double)x / b[i]))-(a + 1);
        if (b[i]>0)
            cnt+=n-((upper_bound(a+1, a + n + 1, floor((double)x/b[i])))-(a+1));
    }
    return cnt<=k;
}
int main(){
    ios::sync_with_stdio(false);
    cin>>n>>m>>k;
    int i,j;
    k
 
--;
    for(i=1;i<=n;i++)
        cin>>a[i];
    for(i=1;i<=m;i++)
        cin>>b[i];
    sort(a+1,a+1+n);
    sort(b+1,b+1+m);
    ll l=-1e13,r=1e13;
    while(l<r){
        ll mid=l+r>>1;
        if(check(mid))
            r=mid;
        else
            l=mid+1;
    }
    cout
 
<<l<<endl;
    return 0;
}