hdoj 2222 Keywords Search(AC自动机)

题目链接：http://acm.hdu.edu.cn/showproblem.php?pid=2222

思路分析：该问题为多模式匹配问题，使用AC自动机解决；需要注意的问题是如何统计该待查询的字符串包含的关键字：

假设待查找的字符串为str[0..n]，则str[i…j]可能为某一个关键字；假设当前正在匹配字符str[k],则以str[i..k]为关键字的所有可能

可能的关键字的最后一个字符为str[k]，使用fail指针进行跳转并判断以str[k]结尾的该结点是否为关键字最后一个结点，重复进行该

操作直到回溯到根节点。

代码如下：

#include <cstdio>
#include <cstring>
#include <iostream>
using namespace std;

const int KIND = 26;
const int LEN_STR = 50 + 10;
const int MAX_N = 1000000 + 10;

struct Node;
Node *q[MAX_N];
char insert_str[LEN_STR];
char str[MAX_N];

struct Node {
    Node *fail;
    Node *next[KIND];
    int count;

    Node()
    {
        fail = NULL;
        count = 0;
        memset(next, NULL, sizeof(next));
    }
};

void Insert(char *str, Node *root)
{
    Node *p = root;
    int i = 0, index = 0;

    while (str[i]) {
        index = str[i++] - ‘a‘;
        if (p->next[index] == NULL)
            p->next[index] = new Node();
        p = p->next[index];
    }
    p->count++; // 单词的末尾会被标记为1
}

void BuildAcAutomation(Node *root)
{
    int head = 0, tail = 0;

    root->fail = NULL;
    q[tail++] = root;
    while (head != tail) {
        Node *temp = q[head++];
        Node *p = NULL;

        for (int i = 0; i < KIND; ++i) {
            if (temp->next[i]) {
                if (temp == root)
                    temp->next[i]->fail = root;
                else {
                    p = temp->fail;
                    while (p != NULL) {
                        if(p->next[i]) {
                            temp->next[i]->fail = p->next[i];
                            break;
                        }
                        p = p->fail;
                    }
                    if (p == NULL)
                        temp->next[i]->fail = root;
                }
                q[tail++] = temp->next[i];
            }

        }
    }
}

int Query(char *str, Node *root)
{
    int i = 0, cnt = 0, index = 0;
    Node *p = root;

    while (str[i]) {
        index = str[i] - ‘a‘;
        while (!p->next[index] && p != root)
            p = p->fail;
        p = p->next[index];
        p = (p == NULL) ? root : p;

        Node *temp = p;
        while (temp != root && temp->count != -1) {
            cnt += temp->count;
            temp->count = -1;
            temp = temp->fail;
        }
        ++i;
    }
    return cnt;
}

int main()
{
    int case_times = 0;
    int ans = 0, n = 0;

    scanf("%d", &case_times);
    while (case_times--) {
        Node *root = new Node();
        scanf("%d", &n);
        while (n--) {
            scanf("%s", insert_str);
            Insert(insert_str, root);
        }
        BuildAcAutomation(root);

        scanf("%s", &str);
        ans = Query(str, root);
        printf("%d\n", ans);
    }
    return 0;
}