CodeForces 933A A Twisty Movement (DP)

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

#define debug puts("YES");
#define rep(x,y,z) for(int (x)=(y);(x)<=(z);(x)++)
#define per(x,y,z) for(int (x)=(y);(x)>=(z);(x)--)

#define ll long long

#define lrt int l,int r,int rt
#define lson l,mid,rt<<1
#define rson mid+1,r,rt<<1|1
#define root l,r,rt
#define mst(a,b) memset((a),(b),sizeof(a))
#define pii pair<int,int>
#define fi first
#define se second
#define mk(x,y) make_pair(x,y)
const int mod=1e9+7;
const int maxn=2e3+10;
const int maxm=2e3+10;
const int ub=1e6;
ll powmod(ll x,ll y){ll t; for(t=1;y;y>>=1,x=x*x%mod) if(y&1) t=t*x%mod; return t;}
ll gcd(ll x,ll y){
    if(y==0) return x;
    return gcd(y,x%y);
}
int dp1[2][maxn],dp2[2][maxn],dp[maxn][maxn][2][2];
int n,a[maxn],ans=0;
int main(){
    cin>>n;
    rep(i,1,n) {cin>>a[i];a[i]--;}
    rep(i,1,n){
        dp1[0][i]=dp1[0][i-1];
        dp1[1][i]=dp1[1][i-1];
        if(a[i]) dp1[1][i]=max(dp1[0][i-1],dp1[1][i-1])+1;
        else dp1[0][i]=dp1[0][i-1]+1;
    }
    per(i,n,1){
        dp2[0][i]=dp2[0][i+1];
        dp2[1][i]=dp2[1][i+1];
        if(a[i]) dp2[1][i]=dp2[1][i+1]+1;
        else dp2[0][i]=max(dp2[0][i+1],dp2[1][i+1])+1;
    }
    rep(i,1,n) rep(j,i,n){
        if(i==j){
            dp[i][j][a[i]][a[i]]=1;
        }else{
            rep(p,0,1) rep(q,p,1) dp[i][j][p][q]=dp[i][j-1][p][q];
            if(a[j])
                dp[i][j][1][1]=dp[i][j-1][1][1]+1;
            else{
                dp[i][j][0][0]=dp[i][j-1][0][0]+1;
                dp[i][j][0][1]=max(dp[i][j-1][0][1],dp[i][j-1][1][1])+1;
            }
        }
        ans=max(ans,dp[i][j][0][0]+dp1[0][i-1]+max(dp2[0][j+1],dp2[1][j+1]));
        ans=max(ans,dp[i][j][0][1]+dp2[1][j+1]+dp1[0][i-1]);
        ans=max(ans,dp[i][j][1][1]+dp2[1][j+1]+max(dp1[0][i-1],dp1[1][i-1]));
    }
    cout<<ans;
    return 0;
}