正向求最長上升子序列並且將記錄，反向求最長上升子序列並且維護答案。

#include<bits/stdc++.h>
using namespace std;
#define rep(i,j,k) for(int i=j;i<=k;i++)
typedef long long ll;

int dp1[500005];
int dp2[500005];
int a[500005];
int tmp[500005];
int n ;

const int INF =1e9;

int ans;

void solve(){
    fill(dp1,dp1+n,INF);
    rep(i,0,n-1) {
        *upper_bound(dp1,dp1+n,a[i])=a[i];
        tmp[i]=lower_bound(dp1,dp1+n,INF)-dp1;
    }
    fill(dp2,dp2+n,INF);
    for(int i=n-1;i>=0;i--){
        *upper_bound(dp2,dp2+n,a[i])=a[i];
        int k=lower_bound(dp2,dp2+n,INF)-dp2;
        ans=max(ans,tmp[i]+k-1);
    }
}

int main(){

    cin>>n;
    rep(i,0,n-1) scanf("%d",&a[i]);
    solve();
    cout<<ans<<endl;
    return 0;
}