好题怎么能没有题解呢？

#include<iostream>
#include<cmath>
#include<algorithm>
#include<cstdio>
using namespace std;
typedef long long LL;
const int MAXN = 1e6 + 10;
const int INF = 0x3f;
int a[MAXN];
int qian[MAXN];
int main()
{
	int n;
	cin >> n;
	int Max = n;
	for(int i = 1;i <= n; i++){
		cin >> a[i];
		if(a[i] == 2){
			qian[i] = qian[i - 1] + 1;
		}
		else{
			qian[i] = qian[i - 1];
		}
	}
	for(int i = 1;i <= n; i++){
		Max = min(Max, 2 * qian[i] + n - i - qian[n]);
	}
	if(Max == n){
		cout << 0;
	}
	else cout << Max;
    return 0;
}