怎么？不能用优化！？

rt

#include<bits/stdc++.h>
#define endl '\n'
using namespace std;

const int N = 5009;

int n, a[N], dp[N], len;

int main() {
    ios::sync_with_stdio(false); cin.tie(0);
    cin >> n;
    for (int i = 1; i <= n; i++) {
        cin >> a[i];
    }
    dp[1] = a[1];
    len = 1;
    for (int i = 1; i <= n; i++) {
        if (a[i] > dp[len]) {
            dp[++len] = a[i];
        } else if (a[i] < dp[len]) {
            int p = upper_bound(dp + 1, dp + 1 + len, a[i]) - dp;
            dp[p] = a[i];
        }
    }
    cout << len << endl;
    return 0;
}