真是简单易懂啊！马上就会了

#include <bits/stdc++.h>
using namespace std;
int n, s1 = 0, s2 = 0, s3 = 0, s4 = 0;
int main() {
    scanf("%d", &n);
    while (n--) {
        int x;
        scanf("%d", &x);
        if (x == 1) {
            s1++;
            s3 = max(s3 + 1, s2 + 1);
        } else {
            s2 = max(s1 + 1, s2 + 1);
            s4 = max(s3 + 1, s4 + 1);
        }
    }
    printf("%d\n", max(s3, s4));
    return 0;
}