It's very easy. Let's watch the AC code!

#include<bits/stdc++.h>
using namespace std;
int n,a[10005],f[10005],ans;
int main(){
	ios::sync_with_stdio(false);
	cin.tie(0);
	cout.tie(0);
	cin>>n;
	for(int i=1;i<=n;i++){
		cin>>a[i];
		f[i]=1;
	}
	for(int i=1;i<=n;i++){
		for(int j=1;j<i;j++){
			if(a[j]<a[i]){
				f[i]=max(f[i],f[j]+1);
			}
		}
	}
	for(int i=1;i<=n;i++){
		ans=max(ans,f[i]);
	}
	cout<<ans;
	return 0;
}

If you have a better way to solve this problem, please let me know. We can learn from each other. Thanks!