题解

#include <bits/stdc++.h>
using namespace std;

int a[200005];
int b[5];

int main() {
	int n;
	cin >> n;
	for (int i = 1; i <= n; i ++) {
		cin >> a[i];
		b[a[i]] ++;
	}
	int x = b[1] + 1, y = n;
	int cnt = 0;
	for (int i = 1; i <= b[1]; i ++) {
		if (a[i] == 2) {
			while (a[x] != 1) {
				x ++;
			}
			swap(a[i], a[x]);
			cnt ++;
		}
		else if (a[i] == 3) {
			while (a[y] != 1) {
				y --;
			}
			swap(a[i], a[y]);
			cnt ++;
		}
	}
	for (int i = b[1] + 1; i <= b[1] + b[2]; i ++) {
		if (a[i] == 3) {
			cnt ++;
		}
	}
	cout << cnt << endl;
    return 0;
}