//题意：给定n个字符串，让你求出在大于n/2个字符串中的最长公共串。//height数组，存的是排名相邻的两个后缀的最长公共前缀。height是按后缀的字典序来的//rank保存的是suffix(i)在所有后缀中从小到大的排列的名次。//后缀数组sa:将str的n个后缀从小到大进行排序之后把排好序的后缀的开头位置顺次放入SA中。#include<stdio.h>#include<string.h>#define max 101000int next[max],sa[max];int wa[max],wb[max],wv[max],ws[max];int rank[max],height[max];int vis[1001];char tem[1001];int str[max],lcp[max],n;char ans[max];int cmp(int *r,int a,int b,int l){ return r[a]==r[b]&&r[a+l]==r[b+l];}void da(int *r,int *sa,int n,int m){ int i,j,p,*x=wa,*y=wb,*t; for(i=0; i<m; i++) ws[i]=0; for(i=0; i<n; i++) ws[x[i]=r[i]]++; for(i=1; i<m; i++) ws[i]+=ws[i-1]; for(i=n-1; i>=0; i--) sa[--ws[x[i]]]=i; for(j=1,p=1; p<n; j*=2,m=p) { for(p=0,i=n-j; i<n; i++) y[p++]=i; for(i=0; i<n; i++) if(sa[i]>=j)y[p++]=sa[i]-j; for(i=0; i<n; i++)wv[i]=x[y[i]]; for(i=0; i<m; i++)ws[i]=0; for(i=0; i<n; i++)ws[wv[i]]++; for(i=1; i<m; i++) ws[i]+=ws[i-1]; for(i=n-1; i>=0; i--) sa[--ws[wv[i]]]=y[i]; for(t=x,x=y,y=t,p=1,x[sa[0]]=0,i=1; i<n; i++) x[sa[i]]=cmp(y,sa[i-1],sa[i],j)?p-1:p++; } return ;}void calheight(int *r,int *sa,int n){ int i,j,k=0; for(i=1; i<=n; i++) rank[sa[i]]=i; for(i=0; i<n; height[rank[i++]]=k) for(k?k--:0,j=sa[rank[i]-1]; r[i+k]==r[j+k]; k++); return ;}int check(int mid,int len){ int count=0; memset(vis,0,sizeof(vis)); for(int i=2; i<=len; i++) { if(height[i]<mid) { memset(vis,0,sizeof(vis)); count=0; continue; }//避免计算重复。 if(!vis[lcp[sa[i]]]) vis[lcp[sa[i]]]=1,count++; if(!vis[lcp[sa[i-1]]]) vis[lcp[sa[i-1]]]=1,count++; if(count>n/2) return 1; } return 0;}void output(int mid,int len){ int count=0,flag=0; memset(vis,0,sizeof(vis)); for(int i=2; i<=len; i++) { if(height[i]<mid) { memset(vis,0,sizeof(vis)); count=0; flag=0; continue; } if(!vis[lcp[sa[i]]]) vis[lcp[sa[i]]]=1,count++; if(!vis[lcp[sa[i-1]]]) vis[lcp[sa[i-1]]]=1,count++; if(count>n/2&&!flag) { for(int j=0; j<mid; j++) ans[j]=str[sa[i]+j]+'a'-1; ans[mid]='\0'; printf("%s\n",ans); flag=1; } } return ;}int main(){ int len; int min; int sumLen; while(scanf("%d",&n),n) { memset(str,'\0',sizeof(str)); min=max; sumLen=0; int m=28; for(int i=0; i<n; i++)//将字符串合并，记录最小长度值 { scanf("%s",tem); len=strlen(tem); if(min>len) min=len; for(int j=0; j<len; j++) { str[sumLen]=tem[j]-'a'+1; lcp[sumLen++]=i; } str[sumLen]=m; lcp[sumLen++]=m++; } str[sumLen]=0; da(str,sa,sumLen+1,m); calheight(str,sa,sumLen); int left=0,right=min,mid; int flag=0; while(right>=left)//二分 { mid=(right+left)/2; if(check(mid,sumLen)) { left=mid+1; flag=mid; } else { right=mid-1; } } if(flag) { output(flag,sumLen); } else printf("?\n"); printf("\n"); } return 0;}
