51Nod 1105 第K大的数 二分答案

#include<iostream>  
#include<algorithm>
#include<string>
#include<string.h>
typedef long long ll;
using namespace std;
const int MAX = 5e4 + 5;
ll n, k, a[MAX], b[MAX];
bool C(ll mid)
{
    ll j = n, cnt = 0;
    for (ll i = 1; i <= n; i++)
    {
        while (j > 0)
        {
            if (a[i] * b[j] > mid)
                j--;
            else break;
        }
        cnt += j;
    }
    return cnt >= k;
}
int main()
{
    ios::sync_with_stdio(false);
    cin >> n >> k;
    k = n*n - k+1;
    for (int i = 1; i <= n; i++)
        cin >> a[i]>> b[i];
    sort(a+1, a + n+1);
    sort(b+1, b + n+1);
    ll l = a[1] * b[1], r = a[n] * b[n];
    for(int i=0;i<100;i++)
    {
        ll mid = (l + r) >> 1;
        if (C(mid)) r = mid;
        else l = mid;
    }
    cout << r << endl;
    return 0;
}