934C - A Twisty Movement - CodeForces Solution

#include <bits/stdc++.h>
#define ll long long
#define mod 998244353
using namespace std;
inline void read(int &x)
{int f=0; x=0;
char c=getchar();
while (c<'0'||c>'9')
{if (c=='-') f=1;
c=getchar();
}
while (c>='0'&&c<='9') x=x*10+c-'0',c=getchar();
if (f) x=-x;
}
int a[2005],n,s1[2005],s2[2005],ans;
int main (){
	int i,j,k;
	read(n);
	for(i=1;i<=n;i++){
		read(a[i]);
		s1[i]=s1[i-1]+(a[i]==1);
		s2[i]=s2[i-1]+(a[i]==2);
	}
	for(i=1;i<=n;i++){
		int sa=0,sb=0;
		for(j=i;j>=1;j--)
			sa=max(sa,s1[j]+s2[i]-s2[j-1]);
		for(j=i;j<=n;j++)
			sb=max(sb,s1[j]-s1[i]+s2[n]-s2[j]);
		ans=max(ans,sa+sb);
	}
	printf("%d",ans);
}/*1697936166.2056048*/