WA on #22 ，求hsck 题目在这

#include<bits/stdc++.h>
using namespace std;
const int N=1e9;
int n,x,ans;
priority_queue<int> q;
map<int,int> mp;
int main()
{
//	freopen("date.in","r",stdin);
//	freopen("x.out","w",stdout);
	cin>>n;
	for(int i=1;i<=n;i++)
	{
		cin>>x;
		mp[x]++;
		if(mp[x]>=2) q.push(N+1);
		else q.push(x);
	}
	for(int i=1;i<=N;i++)
	{
		if(mp[i]) continue;
		if(q.size()>=2)
		{
			if(q.top()>i)
			{
				mp[q.top()]--;
				q.pop();
			}
			else
			{
				ans=i-1;
				break;
			}
			if(q.top()>i)
			{
				mp[q.top()]--;
				q.pop();
			}
			else
			{
				ans=i-1;
				break;
			}
		}
		else
		{
			ans=i-1;
			break;
		}
		if(i==N) ans=N;
	}
	cout<<ans<<"\n";
	return 0;
}