10天前交的题，当时OJ挂了，现在才想起去看结果。
如果在状态上深搜的过程中出现了环，说明可以无限长。
//#pragma comment(linker, "/STACK:1024000000,1024000000") #include<cstdio> #include<cstring> #include<cstdlib> #include<algorithm> #include<iostream> #include<sstream> #include<cmath> #include<climits> #include<string> #include<map> #include<queue> #include<vector> #include<stack> #include<set> using namespace std; typedef long long ll; typedef unsigned long long ull; typedef pair<int,int> pii; #define pb(a) push_back(a) #define INF 0x1f1f1f1f #define lson idx<<1,l,mid #define rson idx<<1|1,mid+1,r #define PI 3.1415926535898 template<class T> T min(const T& a,const T& b,const T& c) { return min(min(a,b),min(a,c)); } template<class T> T max(const T& a,const T& b,const T& c) { return max(max(a,b),max(a,c)); } void debug() { #ifdef ONLINE_JUDGE #else freopen("in.txt","r",stdin); // freopen("d:\out1.txt","w",stdout); #endif } int getch() { int ch; while((ch=getchar())!=EOF) { if(ch!=' '&&ch!=' ')return ch; } return EOF; } const int MAX_NODE=500000; const int SIGMA_SIZE=26; int ch[MAX_NODE][SIGMA_SIZE]; int fail[MAX_NODE]; int val[MAX_NODE]; char dmc[MAX_NODE]; int sz; int idx(char c){return c-'A';} void init() { sz=1; memset(ch[0],0,sizeof(ch[0])); val[0]=0; } void insert(const char *s) { int u=0; for(int i=0;s[i]!=' ';i++) { int c=idx(s[i]); if(!ch[u][c]) { memset(ch[sz],0,sizeof(ch[sz])); val[sz]=0; dmc[sz]=s[i]; ch[u][c]=sz++; } u=ch[u][c]; } val[u]=1; } void construct() { queue<int> q; fail[0]=0; for(int c=0;c<SIGMA_SIZE;c++) { int u=ch[0][c]; if(u){fail[u]=0;q.push(u);} } while(!q.empty()) { int r=q.front();q.pop(); for(int c=0;c<SIGMA_SIZE;c++) { int u=ch[r][c]; if(!u){ch[r][c]=ch[fail[r]][c];continue;} q.push(u); int v=fail[r]; while(v&&!ch[v][c])v=fail[v]; fail[u]=ch[v][c]; val[u]|=val[fail[u]]; } } } int n,m; int vis[MAX_NODE]; int dp[MAX_NODE]; int road[MAX_NODE]; int flag; int f(int u) { if(vis[u]==-1) { flag=0;return 0; }else if(vis[u]==1) return dp[u]; vis[u]=-1; dp[u]=0; for(int c=n-1;c>=0;c--) { int v=ch[u][c]; if(!val[v]) { if(f(v)+1>dp[u]) { dp[u]=f(v)+1; road[u]=c; } } if(!flag)return 0; } vis[u]=1; return dp[u]; } int main() { // debug(); int t; scanf("%d",&t); for(int ca=1;ca<=t;ca++) { scanf("%d%d",&n,&m); init(); for(int i=1;i<=m;i++) { char buf[100]; scanf("%s",buf); insert(buf); } construct(); memset(vis,0,sizeof(vis)); flag=1; int num=f(0); if(!flag||!num)printf("No "); else { int u=0; while(num--) { printf("%c",road[u]+'A'); u=ch[u][road[u]]; } printf(" "); } // printf("%d %d ",num,flag); } return 0; }