2016夏季练习——二分

来源：POJ3579

两次二分

代码：

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cstring>
using namespace std;
const int MAXN = 100000+10;
int n;
int a[MAXN];
bool judge(int m){
	int cnt=0;
	for(int i=0;i<n;++i){
		cnt+=n-(lower_bound(a,a+n,a[i]+m)-a);
	}
	//cout<<"infun: "<<cnt<<endl;
	if(cnt>(n-1)*n/4) return 1;
	else return 0;
}
int main(){
	while(scanf("%d",&n)!=EOF){
		for(int i=0;i<n;i++){
			scanf("%d",a+i);
		}
		//cout<<"des: "<<n*(n-1)/4<<endl;
		sort(a,a+n);
		int left=0;
		int right=a[n-1]-a[0];
		int mid,ans;
		while(left<=right){
			mid=(left+right)/2;
			//cout<<"pro: "<<left<<" "<<mid<<" "<<right<<endl;
			if(judge(mid)){
				left = mid+1;
				ans=mid;
			}
			else{
				right=mid-1;
			}
		}
		//cout<<left<<endl;
		//cout<<mid<<endl;
		cout<<right<<endl;
		//cout<<ans<<endl;
	}
	return 0;
}