数据结构之$\color{#32CD32}{AC自动机}$

解决的问题

• 插入给定的 n 个长度不超过 50 的由小写英文字母组成的单词
• 输出给定文章中包含单词的个数

$AC自动机相当于在trie树上对于每个节点预处理出next[i]$

$在对文章进行搜索时,每次失败就跳到next[i]的地方,直到匹配成功$

首先对给定的单词插入trie

$Code$

void insert()
{
    int  p = 0;
    for (int i = 0; i < str[i]; i ++ )
    {
        int c = str[i] - 'a';
        if (!trie[p][c])trie[p][c] = ++tot;
        p = trie[p][c];
    }

    cnt[p]++;
}

$然后,用bfs递推出所有next的值$

$next[i] 表示 i号节点结尾的字符串的最大后缀(后最必须在树上能表示出)$

$根据父亲节点的next值,就能顺利推出公式:next[j] = trie[next[i]][ch] (j是i沿字符指针ch指向的儿子节点)$

$特别的,因为next[i]指向的是i的对应后缀,trie[next[i]][ch]可能为空$

$这样导致next[i]只能指向第二大的值,所以在遍历trie[next[i]][ch]时$

$给他特定赋值为父节点的next值的ch指针$

$Code$

void build()
{
    int hh = 0, tt = -1;

    for (int i = 0; i < 26; i ++ )
        if (trie[0][i])
            q[++tt] = trie[0][i];

    while (hh <= tt)
    {
        int t = q[hh++];
        for (int i = 0; i < 26; i ++ )
        {
            int p = trie[t][i];
            if (!p)trie[t][i] = trie[ne[t]][i];
            else
            {
                ne[p] = trie[ne[t]][i];
                q[++tt] = p;
            }
        }
    }
}

$最后,在查询文章时,枚举每一位字母$

$查找以当前字母为后缀的单词数量,就能统计出文章中单词出现的总数$

$Code$

    int res = 0;
    for (int i = 0, j = 0; str[i]; i ++ )
    {
        int c = str[i] - 'a';
        j = trie[j][c];

        int  p = j;
        while (p)
        {
            res += cnt[p];
            p = ne[p];
        }
    }

$\color{#32CD32}{AC自动机Code}$

#include<bits/stdc++.h>

using namespace std;

const int N = 10010, M = 1000010, S = 55;

int n;
char str[M];
int trie[N * S][26], cnt[N * S], tot;
int q[N * S], ne[N * S];

void insert()
{
    int p = 0;
    for (int i = 0; i < str[i]; i ++ )
    {
        int c = str[i] - 'a';
        if (!trie[p][c])trie[p][c] = ++tot;
        p = trie[p][c];
    }

    cnt[p]++;
}

void build()
{
    int hh = 0, tt = -1;

    for (int i = 0; i < 26; i ++ )
        if (trie[0][i])
            q[++tt] = trie[0][i];

    while (hh <= tt)
    {
        int t = q[hh++];
        for (int i = 0; i < 26; i ++ )
        {
            int p = trie[t][i];
            if (!p)trie[t][i] = trie[ne[t]][i];
            else
            {
                ne[p] = trie[ne[t]][i];
                q[++tt] = p;
            }
        }
    }
}


int main()
{
    scanf("%d", &n);
    for (int i = 1; i <= n; i ++ )
    {
        scanf("%s", str);
        insert();
    }

    build();

    scanf("%s", str);

    int res = 0;
    for (int i = 0, j = 0; str[i]; i ++ )
    {
        int c = str[i] - 'a';
        j = trie[j][c];

        int  p = j;
        while (p)
        {
            res += cnt[p];
            p = ne[p];
        }
    }

    printf("%d\n", res);

    return 0;
}