60分代码：

#include <bits/stdc++.h>
using namespace std;
constexpr int N = 1e5 + 5;
int a[N], f[N];
int main()
{
    int n;
    scanf("%d", &n);
    for (int i = 1; i <= n; i++)
        scanf("%d", &a[i]);
    memset(f, 0, sizeof(f));
    for (int i = 1; i <= n; i++)
    {
        f[0] = f[0] + (a[i] == 0 ? 1 : 0);
        for (int j = i; j >= 1; j--)
            f[j] = max(f[j] + (a[i] == i - j ? 1 : 0), f[j - 1]);
    }
    int ans = 0;
    for (int i = 1; i <= n; i++)
        ans = max(ans, f[i]);
    printf("%d\n", ans);
    return 0;
}

60分代码：

#include <bits/stdc++.h>
using namespace std;
constexpr int N = 1e5 + 5;
int a[N], f[N];
int main()
{
	int n;
	scanf("%d", &n);
	for (int i = 1; i <= n; i++)
		scanf("%d", &a[i]);
	memset(f, 0, sizeof(f));
	for (int i = 1; i <= n; i++)
	{
		f[0]=f[0] + (a[i] == 0 ? 1 : 0);
		for (int j = i; j >= 1; j--)
			f[j] = max(f[j] + (a[i] == i - j ? 1 : 0), f[j - 1]);
	}
	int ans=0;for(int i=1;i<=n;i++) ans=max(ans,f[i]);
	printf("%d\n", ans);
	return 0;
}