POJ 3579解题记录

#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cstring>
#include<functional>
#include<queue>
using namespace std;
typedef long long ll;
const int maxv=1e5+30;
ll n;
ll tol;
ll a[maxv];
bool check(ll mid){
    ll num=0;
    for(int i=0;i<n;i++){ 
        if(a[i]+mid>a[n-1]) break;
        ll *p=lower_bound(a+i,a+n,a[i]+mid);
        num+=n-(p-a);
    }
    if(num>(tol/2)) return 1;
    else return 0;
}
int main(){
    freopen("in","r",stdin);
    while(cin>>n){
        tol=n*(n-1)/2;
        for(int i=0;i<n;i++) scanf("%lld",&a[i]);
        sort(a,a+n);
        ll l=0,r=1e9;
        while(r-l>1){
            ll mid=(r+l)/2;
            if(check(mid)) l=mid;
            else r=mid;
        }
        if(check(l)) cout<<l<<endl;
        else cout<<r<<endl;
    }
    return 0;
}