ABC E双指针求条

#include<bits/stdc++.h>
#define int long long
using namespace std;
const int N=5e5+5;

int a[N],n;
int vis[N];

signed main()
{
	ios::sync_with_stdio(false);
	cin.tie(0);
	cout.tie(0);
	cin>>n;
	for(int i=1;i<=n;i++)
	{
		cin>>a[i];
	}
	int ans=0;
	int l=0,r=2;
	while(1)
	{
		l++;
		while(vis[l])
		{
			l++;
		}
		if(l>n) break;
		int num=a[l]*2;
		while(a[r]<num||vis[r])
		{
			r++;
			if(r>n) break;
		}
		if(r>n) break;
		vis[l]=1;
		vis[r]=1;
		ans++;
	}
	cout<<ans<<endl;
	return 0;
}