51Nod 1105 第K大的数 二分答案

很典型的二分答案的题目 码着

#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;
}