K`th number

二分答案，check()用two point写

#include<iostream>#include<cstdio>#include<algorithm>#define LL long long#define N 200009using 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;}