令\(dp_{i, j}\) 代表长度为 \(i\),结尾是 \(j\) 的最少操作次数 \((i \in [1,n], j \in [-1, 1])\)

\[
\begin{gathered}
dp_{i,j} = \begin{cases}
a_i = -1& \begin{cases}
dp_{i, -1} = dp_{i - 1, -1} \cr
dp_{i, 0} = +\infty \cr
dp_{i, 1} = dp_{i - 1, 1} + 2 \cr
\end{cases} \cr
a_i = 0 & \begin{cases}
dp_{i, -1} = dp_{i - 1, -1} + 1 \cr
dp_{i, 0} = \min(dp_{i - 1, -1}, dp_{i - 1, 0}) \cr
dp_{i, 1} = dp_{i - 1, 1} + 1 \cr
\end{cases} \cr
a_i = 1 & \begin{cases}
dp_{i, -1} = dp_{i - 1, -1} + 2 \cr
dp_{i, 0} = dp_{i - 1, -1} + 1 \cr
dp_{i, 1} = min(dp_{i - 1, -1}, dp_{i - 1, 0}, dp_{i - 1, 1}) \cr
\end{cases} \cr
\end{cases}
\end{gathered}
\]

因为数组下标不能为 \(-1\)，我们可以将 \(-1, 0, 1\) 替换为 \(0, 1, 2\)。

std如下：

#include <iostream>
#include <cstring>
using namespace std;
const int N = 1000010;
const int INF = 0x3f3f3f3f;

int n;
int a[N];
int dp[N][3];

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];

    memset(dp, INF, sizeof(dp));
    dp[1][a[1] + 1] = 0;
    for (int i = 2; i <= n; i++) {
        if (a[i] == -1) {
            dp[i][0] = dp[i - 1][0];
            dp[i][2] = dp[i - 1][2] + 2;
        } else if (a[i] == 0) {
            dp[i][0] = dp[i - 1][0] + 1;
            dp[i][1] = min(dp[i - 1][0], dp[i - 1][1]);
            dp[i][2] = dp[i - 1][2] + 1;
        } else if (a[i] == 1) {
            dp[i][0] = dp[i - 1][0] + 2;
            dp[i][1] = dp[i - 1][0] + 1;
            dp[i][2] = min(dp[i - 1][0], min(dp[i - 1][1], dp[i - 1][2]));
        }
    }
    int ans = min(dp[n][0], min(dp[n][1], dp[n][2]));
    if (ans == INF) cout << "Baka\n";
    else cout << ans << '\n';
    return 0;
}