HDU-2222-Keywords Search(AC自动机模板)

Posted 2021-02-08 angel-demon

这题是AC自动机的模板题，AC自动机是结合了字典树，和KMP两种算法产生的，去年为了学习AC自动机去看了前面说的两种算法，但是可能因为KMP当时理解的不够透彻所以这题当时也只是半背代码的做出来了，没多久就忘了。经过那么久的学习感觉KMP掌握的差不多了，今天回顾了一下AC自动机，并记录一下模板

• AC自动机

  Accepted

  2222

  327MS

  56848K

  1991 B

  G++

  #include "cstdio"
  #include "cstring"
  #include "cstdlib"
  #include "queue"
  using namespace std;
  const int MAXS = 1000005;
  const int MAXP = 55;
  const int SIZE = 26;
  struct Tree {
      int tail;
      Tree* next[SIZE];
      Tree* fail;
  }* root;
  char p[MAXP], s[MAXS];
  Tree* init() {
      Tree* point = (Tree*)malloc(sizeof(Tree));
      point->tail = 0;
      memset(point->next, NULL, sizeof(point->next));
      point->fail = NULL;
      return point;
  }
  void insert(char* p) {
      Tree* point = root;
      for (int i = 0; p[i]; i++) {
          int index = p[i] - ‘a‘;
          if (point->next[index] == NULL) {
              point->next[index] = init();
          }
          point = point->next[index];
      }
      point->tail++;
  }
  void build() {
      queue<Tree*> q;
      Tree* point;
      Tree* p1;
      Tree* p2;
      q.push(root);
      while (!q.empty()) {
          point = q.front();
          q.pop();
          for (int i = 0; i < SIZE; i++) {
              if (point->next[i] != NULL) {
                  p1 = point->fail;
                  p2 = point->next[i];
                  while (p1 != NULL) {
                      if (p1->next[i] != NULL) {
                          p2->fail = p1->next[i];
                          break;
                      }
                      p1 = p1->fail;
                  }
                  if (p1 == NULL) {
                      p2->fail = root;
                  }
                  q.push(p2);
              }
          }
      }
  }
  int query(char* s) {
      int cnt = 0;
      Tree* p1;
      Tree* p2;
      p1 = root; 
      for (int i = 0; s[i]; i++) {
          int index = s[i] - ‘a‘;
          while (p1 != root && p1->next[index] == NULL) {
              p1 = p1->fail;
          }
          p1 = p1->next[index];
          if (p1 == NULL) {
              p1 = root;
          }
          p2 = p1;
          while (p2 != root && p2->tail != -1) {
              cnt += p2->tail;
              p2->tail = -1;
              p2 = p2->fail;
          }
      }
      return cnt;
  }
  void clear(Tree* point) {
      for (int i = 0; i < SIZE; i++) {
          if (point->next[i] != NULL) {
              clear(point->next[i]);
          }
      }
      free(point);
  }
  int main() {
      int t, n;
      scanf("%d", &t);
      while (t--) {
          root = init();
          scanf("%d", &n);
          while (n--) {
              scanf("%s", p);
              insert(p);
          }
          scanf("%s", s);
          build();
          printf("%d
  ", query(s));
          clear(root);
          root = NULL;
      }
      return 0;
  } 

  build的时候网上的代码都是用自己写的队列，我一开始用自己写的队列运行时间达到了171ms，确实比库里封装的快不少，但是我还是喜欢用库里的比较保险。弄懂了KMP和字典树看一下网上的讲解就好理解了。