HDU 2222 Keywords Search（AC自动机）

题目连接：http://acm.hdu.edu.cn/showproblem.php?pid=2222

题意：给你n个模式串，一个长为m的主串，问有多少个模式串在主串中出现了（模式串可能重复）

思路：AC自动机水题，先根据这n个模式串建立trie树，再构造fail指针，最后扫描主串即可。

code：

#include <cstdio>
#include <cstring>
#include <queue>
using namespace std;
const int KIND = 26;

struct node
{
    int num;
    node* fail;
    node* next[KIND];
    node()
    {
        num = 0;
        fail = NULL;
        for (int i = 0; i < KIND; ++i) next[i] = NULL;
    }
};
node* root;

void Insert(char str[])
{
    node* temp = root;
    int len = strlen(str);
    for (int i = 0; i < len; ++i)
    {
        int curr = str[i] - 'a';
        if (temp->next[curr] == NULL)
            temp->next[curr] = new node();
        temp = temp->next[curr];
    }
    ++(temp->num);
}

void Build()
{
    queue<node*> Q;
    root->fail = root;
    for (int i = 0; i < KIND; ++i)
    {
        if (root->next[i] == NULL)
            root->next[i] = root;
        else
        {
            root->next[i]->fail = root;
            Q.push(root->next[i]);
        }
    }
    while (!Q.empty())
    {
        node* temp = Q.front();
        Q.pop();
        for (int i = 0; i < KIND; ++i)
        {
            if (temp->next[i] == NULL)
                temp->next[i] = temp->fail->next[i];
            else
            {
                temp->next[i]->fail = temp->fail->next[i];
                Q.push(temp->next[i]);
            }
        }
    }
}

int Query(char str[])
{
    int len = strlen(str);
    node* temp = root;
    int res = 0;
    for (int i = 0; i < len; ++i)
    {
        int curr = str[i] - 'a';
        temp = temp->next[curr];
        node* x = temp;
        while (x != root)
        {
            res += x->num;
            x->num = 0;
            x = x->fail;
        }
    }
    return res;
}

char str[1000010];
int main()
{
    int nCase;
    scanf("%d", &nCase);
    while (nCase--)
    {
        int n;
        scanf("%d", &n);
        root = new node();
        for (int i = 0; i < n; ++i)
        {
            scanf("%s", str);
            Insert(str);
        }
        Build();
        scanf("%s", str);
        printf("%d
", Query(str));
    }
    return 0;
}

 数组实现：

#include <cstdio>
#include <cstring>
#include <queue>
using namespace std;
const int KIND = 26;
const int MAXN = 500005;
const int MAXM = 1000005;

struct Trie
{
    int next[MAXN][KIND], fail[MAXN], end[MAXN];
    int root, L;

    int create()
    {
        for (int i = 0; i < KIND; ++i)
            next[L][i] = -1;
        end[L++] = 0;
        return L - 1;
    }

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

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

    void build()
    {
        queue<int>Q;
        fail[root] = root;
        for (int i = 0; i < KIND; ++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 < KIND; ++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 str[])
    {
        int now = root;
        int len = strlen(str);
        int res = 0;
        for (int i = 0; i < len; ++i)
        {
            int curr = str[i] - 'a';
            now = next[now][curr];
            int temp = now;
            while (temp != root)
            {
                res += end[temp];
                end[temp] = 0;
                temp = fail[temp];
            }
        }
        return res;
    }
};

Trie ac;
char str[MAXM];
int main()
{
    int nCase;
    scanf("%d", &nCase);
    while (nCase--)
    {
        int n;
        scanf("%d", &n);
        ac.init();
        for (int i = 0; i < n; ++i)
        {
            scanf("%s", str);
            ac.insert(str);
        }
        ac.build();
        scanf("%s", str);
        printf("%d
", ac.query(str));
    }
    return 0;
}