K`th number

二分答案，check()用two point写

#include<iostream>
#include<cstdio>
#include<algorithm>
#define LL long long
#define N 200009
using namespace std;
int n,m;
LL k;
LL a[N],b[N],num,L,R,mid;
bool flag;
inline int read() //读入优化 
{ 
    int f=1,p=0;char c=getchar(); 
    while(c>'9'||c<'0'){if(c=='-')f=-1;c=getchar();} 
    while(c>='0'&&c<='9'){p=p*10+c-'0';c=getchar();} 
    return f*p; 
}
bool check(LL x)//小于等于 
{
    LL tot=0;
    int t1=1,t2=m;
    while(t1<=n)//two point
    {
        while(a[t1]*b[t2]>x&&t1<=n&&t2>=1) t2--;
        tot+=t2; t1++;
    }
    return tot<k;//注意判定条件！
}
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(a+1,a+n+1);sort(b+1,b+m+1);

    L=1,R=a[n]*b[m];
    while(L<=R)
    {
        mid=(L+R)>>1;
        if(check(mid)) L=mid+1;
        else R=mid-1;
    }
    printf("%lld\n",L);
    return 0;
}