PAM练习
题:https://www.luogu.com.cn/problem/P1659
题意:问前k大的奇数长度的回文串的长度乘积;
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#include<bits/stdc++.h> using namespace std; typedef long long ll; const int M=1e6+6; const int maxn=26; const int mod=19930726; char s[M]; struct node{ ll val,num; bool operator<(const node &b)const{ return val>b.val; } }b[M]; struct pam{ int son[M][maxn],cnt[M],num[M],fail[M],len[M]; char s[M]; int tot,last; int newnode(int Len){ for(int i=0;i<maxn;i++) son[tot][i]=0; cnt[tot]=0; num[tot]=0; fail[tot]=0; len[tot]=Len; return tot++; } void init(){ s[0]='#'; tot=0; last=0; newnode(0); newnode(-1); fail[0]=1; } int getfail(int p,int i){ while(s[i-len[p]-1]!=s[i]) p=fail[p]; return p; } void solve(const char *buf){ init(); int n=strlen(buf+1); for(int i=1;i<=n;i++){ s[i]=buf[i]-'a'; int cur=getfail(last,i); if(!son[cur][s[i]]){ int now=newnode(len[cur]+2); fail[now]=son[getfail(fail[cur],i)][s[i]]; son[cur][s[i]]=now; num[now]=num[fail[now]]++; } cnt[last=son[cur][s[i]]]++; } for(int i=tot-1;i>=0;i--) cnt[fail[i]]+=cnt[i]; } }PAM; ll ksm(ll a,ll b){ ll t=1ll; while(b){ if(b&1){ t=(t*a)%mod; } b>>=1ll; a=(a*a)%mod; } return t; } int main(){ ll n,k; scanf("%lld%lld",&n,&k); scanf("%s",s+1); PAM.solve(s); int m=0; for(int i=0;i<PAM.tot;i++){ // cout<<PAM.len[i]<<"!!"<<PAM.cnt[i]<<endl; if(PAM.len[i]&1){ b[m].val=PAM.len[i]; b[m].num=PAM.cnt[i]; m++; } } sort(b,b+m); ll ans=1ll; for(int i=0;i<m;i++){ ans=(1ll*ans*ksm(b[i].val,min(1ll*b[i].num,k)))%mod; // cout<<ans<<endl; k-=b[i].num; if(k<=0) break; } printf("%lld\n",ans); return 0; }
题:https://codeforces.com/contest/17/problem/E
题意:求不相交回文串的对数
分析:先求整体对数减去相交对数即可
注意:内存限制,fail改成邻接表形式
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#include<iostream> #include<cstdio> #include<cstdlib> #include<cstring> #include<cmath> #include<algorithm> #include<set> #include<map> #include<vector> #include<queue> using namespace std; #define ll long long #define RG register #define MAX 2000020 #define MOD 51123987 int n,p1[MAX],p2[MAX],ans,dep[MAX]; char s[MAX]; struct Line{int v,next,w;}e[MAX]; int cnt=1; int h[MAX]; inline void Add(int u,int v,int w){e[cnt]=(Line){v,h[u],w};h[u]=cnt++;} struct PT { struct Node { int ff,len; }t[MAX]; int tot,last; void init() { for(int i=0;i<=tot;++i) { h[i]=0; t[i].ff=t[i].len=0; } cnt=1; last=0; t[tot=1].len=-1; t[0].ff=t[1].ff=1; } int nt(int k,int c) { for(int i=h[k];i;i=e[i].next) if(e[i].w==c)return e[i].v; return 0; } void extend(int c,int n,char *s) { int p=last; while(s[n-t[p].len-1]!=s[n])p=t[p].ff; if(!nt(p,c)) { int v=++tot,k=t[p].ff; while(s[n-t[k].len-1]!=s[n])k=t[k].ff; t[v].len=t[p].len+2; t[v].ff=nt(k,c); dep[v]=dep[t[v].ff]+1; Add(p,v,c); } last=nt(p,c); } }pt1; int main() { scanf("%d",&n); scanf("%s",s+1); pt1.init(); for(int i=1;i<=n;++i) { pt1.extend(s[i]-97,i,s); ans=(ans+(p1[i]=dep[pt1.last]))%MOD; } ans=1ll*ans*(ans-1)/2%MOD; reverse(&s[1],&s[n+1]); memset(dep,0,sizeof(dep)); pt1.init(); for(int i=1;i<=n;++i) { pt1.extend(s[i]-97,i,s); p2[n-i+1]=dep[pt1.last]; } for(int i=n;i;--i)(p2[i]+=p2[i+1])%=MOD; for(int i=1;i<=n;++i)ans=(ans-1ll*p1[i]*p2[i+1]%MOD+MOD)%MOD; printf("%d\n",ans); return 0; }
题:http://www.lydsy.com/JudgeOnline/problem.php?id=2565
题意:求俩个回文组成的最长串长度。
分析:正方跑一遍PAM就可以计数了
![](https://images.cnblogs.com/OutliningIndicators/ContractedBlock.gif)
#include<bits/stdc++.h> using namespace std; typedef long long ll; const int M=1e5+6; const int maxn=26; char s[M],s1[M]; int a[M]; struct pam{ int son[M][maxn],cnt[M],fail[M],len[M],sum[M]; char s[M]; int tot,last; int newnode(int Len){ for(int i=0;i<maxn;i++) son[tot][i]=0; cnt[tot]=0; fail[tot]=0; len[tot]=Len; return tot++; } void init(){ s[0]='#'; tot=0; last=0; newnode(0); newnode(-1); fail[0]=1; } int getfail(int p,int i){ while(s[i-len[p]-1]!=s[i]) p=fail[p]; return p; } void solve(char buf[]){ init(); int n=strlen(buf+1); for(int i=1;i<=n;i++){ s[i]=buf[i]-'a'; int cur=getfail(last,i); if(!son[cur][s[i]]){ int now=newnode(len[cur]+2); fail[now]=son[getfail(fail[cur],i)][s[i]]; son[cur][s[i]]=now; } cnt[last=son[cur][s[i]]]++; sum[i]=len[last]; } for(int i=tot-1;i>=0;i--) cnt[fail[i]]+=cnt[i]; } }PAM; int main(){ int n; scanf("%s",s+1); n=strlen(s+1); for(int i=n;i>=1;i--) s1[i]=s[n-i+1]; PAM.solve(s); for(int i=1;i<=n;i++) a[i]=PAM.sum[i]; PAM.solve(s1); int ans=2; for(int i=1;i<=n;i++) if(PAM.sum[i]>=1&&a[n-i]>=1) ans=max(ans,PAM.sum[i]+a[n-i]); printf("%d\n",ans); return 0; }