AC自动机

首先简要介绍一下AC自动机：Aho-Corasick automation，该算法在1975年产生于贝尔实验室，是著名的多模匹配算法之一。一个常见的例子就是给出n个单词，再给出一段包含m个字符的文章，让你找出有多少个单词在文章里出现过。要搞懂AC自动机，先得有字典树Trie和KMP模式匹配算法的基础知识。KMP算法是单模式串的字符匹配算法，AC自动机是多模式串的字符匹配算法。

/*
 *  求目标串中出现了几个模式串
 */
struct Trie
{
    int next[500010][26], fail[500010], end[500010];
    int root, L;
    int newnode()
    {
        for (int i = 0; i < 26; i++)
        {
            next[L][i] = -1;
        }
        end[L++] = 0;
        return L - 1;
    }

    void init()
    {
        L = 0;
        root = newnode();
    }

    void insert(char buf[])
    {
        int len = (int)strlen(buf);
        int now = root;
        for (int i = 0; i < len; i++)
        {
            if (next[now][buf[i] - 'a'] == -1)
            {
                next[now][buf[i] - 'a'] = newnode();
            }
            now = next[now][buf[i] - 'a'];
        }
        end[now]++;
    }

    void build()
    {
        queue<int>Q;
        fail[root] = root;
        for (int i = 0; i < 26; i++)
        {
            if (next[root][i] == -1)
            {
                next[root][i] = root;
            }
            else
            {
                fail[next[root][i]] = root;
                Q.push(next[root][i]);
            }
        }
        while (!Q.empty())
        {
            int now = Q.front();
            Q.pop();
            for (int i = 0;i < 26;i++)
            {
                if (next[now][i] == -1)
                {
                    next[now][i] = next[fail[now]][i];
                }
                else
                {
                    fail[next[now][i]]=next[fail[now]][i];
                    Q.push(next[now][i]);
                }
            }
        }
    }

    int query(char buf[])
    {
        int len = (int)strlen(buf);
        int now = root;
        int res = 0;
        for (int i = 0; i < len; i++)
        {
            now = next[now][buf[i] - 'a'];
            int temp = now;
            while (temp != root)
            {
                res += end[temp];
                end[temp] = 0;
                temp = fail[temp];
            }
        }
        return res;
    }

    void debug()
    {
        for (int i = 0; i < L; i++)
        {
            printf("id = %3d,fail = %3d,end = %3d,chi = [", i, fail[i], end[i]);
            for (int j = 0; j < 26; j++)
            {
                printf("%2d", next[i][j]);
            }
            printf("]
");
        }
    }
};

char buf[1000010];
Trie ac;

int main()
{
    int T;
    int n;
    scanf("%d", &T);
    while(T--)
    {
        scanf("%d", &n);
        ac.init();
        for (int i = 0; i < n; i++)
        {
            scanf("%s", buf);
            ac.insert(buf);
        }
        ac.build();
        scanf("%s", buf);
        printf("%d
", ac.query(buf));
    }
    return 0;
}