回文自动机（BZOJ2565）

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

  int p,next[][],cnt[],len[],fail[],last,a[],maxl[],maxr[];
  char st[];

  int newnode(int l){
    p++;
    for (int i=;i<=;i++) next[p][i]=;
    cnt[p]=;
    len[p]=l;
    return(p);
  }

  void init(){
    p=-;
    newnode();
    newnode(-);
    last=;
    st[]=-;
    fail[]=;
  }

  int get_fail(int po,int x,int num){
    while (a[po-len[x]-]!=num) x=fail[x];
    return(x);
  }

  void add(int po,int c){
    int cur=get_fail(po,last,c);
    if (!next[cur][c]){
      int now=newnode(len[cur]+);
      fail[now]=next[get_fail(po,fail[cur],c)][c];
      next[cur][c]=now;
    }
    last=next[cur][c];
    cnt[last]++;
  }

  void count(){
    for (int i=p;i>=;i--) cnt[fail[i]]+=cnt[i];
  }

  int main(){
      scanf("%s",&st);
      int n=strlen(st);
      for (int i=;i<=n;i++) a[i]=st[i-]-'a'+;

      init();
      for (int i=;i<=n;i++)
      add(i,a[i]),maxl[i]=len[last];
      count();

      for (int i=;i<=n/;i++){
        int t=a[i];a[i]=a[n-i+];a[n-i+]=t;
    }
      init();
      for (int i=;i<=n;i++)
      add(i,a[i]),maxr[n-i+]=len[last];
      count();

    int ans=;
      for (int i=;i<n;i++) ans=max(ans,maxl[i]+maxr[i+]);
      printf("%d\n",ans);
  }

每个节点表示一个本质不同的回文串（最多n个）。

进行count()后，cnt中存每个本质不同的回文串的出现次数。

------------------------------------------------------------------------

CODECHEF APRIL LUNCHTIME 2015 PALPROB

在fail树上转移palindromness

#include <cstdio>
#include <cstring>
#include <iostream>
#include <map>
#define LL long long
using namespace std;

  map <int,int> mp,mpb;
  int p,tr[][],cnt[],len[],fail[],last,a[],maxl[],maxr[];
  int ans[],nd[],nxt[],des[],T,scnt;
  char st[];

  void addedge(int x,int y){
      nxt[++scnt]=nd[x];des[scnt]=y;nd[x]=scnt;
  }

  int newnode(int l){
    p++;
    for (int i=;i<=;i++) tr[p][i]=;
    cnt[p]=;ans[p]=;
    len[p]=l;
    return(p);
  }

  void init(){
    p=-;
    newnode();newnode(-);
    last=;
    st[]=-;
    fail[]=;
  }

  int get_fail(int po,int x,int num){
    while (a[po-len[x]-]!=num) x=fail[x];
    return(x);
  }

  void add(int po,int c){
    int cur=get_fail(po,last,c);
    if (tr[cur][c]==){
      int now=newnode(len[cur]+);
      fail[now]=tr[get_fail(po,fail[cur],c)][c];
      tr[cur][c]=now;
    }
    last=tr[cur][c];
    cnt[last]++;
  }

  void count(){
    for (int i=p;i>=;i--) cnt[fail[i]]+=cnt[i];
  }

  void dfs(int po){
      if (len[po]>=){
        int t;
      if (len[po]<=) t=-;else t=len[po]/;
      int p=mp[t];
      if (mpb[t])
        ans[po]=ans[p]+;else
        ans[po]=;
    }

    mpb[len[po]]=;mp[len[po]]=po;
    for (int p=nd[po];p!=-;p=nxt[p])
       dfs(des[p]);
     mpb[len[po]]=;
  } 

  int main(){
    freopen("a.in","r",stdin);

    scanf("%d",&T);
    while (T--){
      scanf("%s",&st);
      int n=strlen(st);
      for (int i=;i<=n;i++) a[i]=st[i-]-'a'+;

      init();
      for (int i=;i<=n;i++)
        add(i,a[i]);
      count();    

      for (int i=;i<=p;i++) nd[i]=-;scnt=;
      for (int i=;i<=p;i++) if (i!=) addedge(fail[i],i);

      mp.clear();mpb.clear();
      dfs();

      LL ret=;
      for (int i=;i<=p;i++) ret+=(LL)ans[i]*cnt[i];
      printf("%lld\n",ret);
    }
  }