B3637 RE求助

code:

#include <bits/stdc++.h>
using namespace std;
const int maxn = 1005;
int n , ans , a[maxn] , f[maxn];
int main()
{
    scanf("%d",&n);
    for(int i = 1 ; i <= n ; i++) cin >> a[i];
    for(int i = 1 ; i <= n ; i++)
    {
        f[i] = 1;
        for(int j = 1 ; j < i ; j++)
        {
            if(a[j] < a[i]) f[i] = max(f[i],f[j]+1);
        }
        ans = max(ans,f[i]);
    }
    printf("%d",ans);
    return 0;
}

请求帮助,为何RE，仅有80分