D - K-th Nearest

原題連結

題解

when you are asked to find the “k-th whatever-est something,” binary search is often adopted as the first step of the solution

以 \(d_k\) 為半徑的線段裡恰好有 \(k\) 個 \(a\)

code

#include<bits/stdc++.h>
#define ll long long
using namespace std;

int a[100005];

void solve()
{
    int n,q;

    cin>>n>>q;

    for(int i=1;i<=n;i++) cin>>a[i];

    sort(a+1,a+1+n);

    while(q--)
    {
        int x,k;
        cin>>x>>k;

        int l=-1,r=2e8+10;

        while(l+1<r)
        {
            int mid=(l+r)/2;

            int p1=upper_bound(a+1,a+1+n,mid+x)-a-1;
            int p2=lower_bound(a+1,a+1+n,x-mid)-a;
            if(p1-p2+1<k) l=mid;
            else r=mid;
        }

        cout<<r<<'\n';
    }
}
int main()
{
    ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);
    int t=1;
    //cin>>t;
    while(t--) solve();
    return 0;
}