字符串---AC自动机

求目标串中出现了几个模式串

思路

一、构建字典树

二、构建fail指针

三、串匹配

例题

HDU2222

#include <stdio.h>
#include <string.h>
#include <queue>
using namespace std;
namespace Trie{
    int next[500010][26], fail[500010], end[500010];//字典树，fail指针，单词末尾标记
    int root, L;//根节点，树节点计数
    int new_node(){
        for(int i = 0; i < 26; i++){
            next[L][i] = -1;
        }
        end[L++] = 0;
        return L-1;
    }
    void init(){
        L=0;
        root = new_node();
    }
    void insert(const char buf[]){//建立字典树
        int len = strlen(buf);
        int now = root;
        for(int i =0; i < len; i++){
            if(next[now][buf[i]-'a'] == -1){//这里默认都是小写字母
                next[now][buf[i]-'a'] = new_node();
            }
            now = next[now][buf[i]-'a'];
        }
        end[now]++;
    }
    void build(){//构建fail指针
        queue<int> que;
        fail[root] = root;
        for(int i = 0; i < 26; i++){
            if(next[root][i] == -1){
                next[root][i] = root;//指向root说明没有
            }else{
                fail[next[root][i]] = root;
                que.push(next[root][i]);
            }
        }
        while(!que.empty()){
            int now = que.front();
            que.pop();
            for(int i = 0; i < 26; i++){
                if(next[now][i] == -1){
                    next[now][i] = next[fail[now]][i]; //没有这个字符就去fail，方便遍历不用判断是否空
                }else{
                    fail[next[now][i]] = next[fail[now]][i]; // 孩子的fail指针，往自己fail方向找
                    que.push(next[now][i]);
                }
            }
        }
    }
    int query(const char buf[]){
        int len = 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;
    }
}
char buf[1000010];
int main(){
    int t,n;
    scanf("%d",&t);
    while(t--){
        scanf("%d",&n);
        Trie::init();
        for(int i=0;i<n;i++){
            scanf("%s",buf);
            Trie::insert(buf);
        }
        Trie::build();
        scanf("%s",buf);
        printf("%d\n",Trie::query(buf));
    }
    return 0;
}