状压dp。状态有点难想,方程十分简单。
#include<iostream> #include<cstdio> #include<cstring> #include<algorithm> #define maxn 455 using namespace std; int t,n,g[maxn][23],f[(1<<22)+5],len,regis[23]; char s[maxn]; void work() { memset(g,0,sizeof(g)); memset(f,0,sizeof(f)); scanf("%d",&n); scanf("%s",s); if (n>21) {printf("NO ");return;} len=strlen(s); for (register int i=1;i<=21;i++) regis[i]=len+1; for (register int i=len-1;i>=0;i--) { for (register int j=1;j<=n;j++) g[i+1][j]=regis[j]; regis[s[i]-'a'+1]=i+1; } for (register int i=1;i<=n;i++) g[0][i]=regis[i]; for (register int i=1;i<=n;i++) g[len][i]=g[len+1][i]=len+1; int top=(1<<n)-1; for (register int i=0;i<=top;i++) for (register int j=1;j<=n;j++) if (i&(1<<(j-1))) f[i]=max(f[i],g[f[i^(1<<(j-1))]][j]); if (f[top]==len+1) printf("NO "); else printf("YES "); } int main() { scanf("%d",&t); for (int i=1;i<=t;i++) work(); return 0; }