#include<iostream>#include<cstring>using namespace std;const int MAX = 100500;const int nMAX = 105;const int mMAX = 1005;int n, sum[nMAX];char str[nMAX][mMAX];int m, num[MAX];int sa[MAX], rank[MAX], height[MAX];int wa[MAX], wb[MAX], wv[MAX], wd[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 n, int m){ int i, j, p, *x = wa, *y = wb, *t; for(i = 0; i < m; i ++) wd[i] = 0; for(i = 0; i < n; i ++) wd[x[i]=r[i]] ++; for(i = 1; i < m; i ++) wd[i] += wd[i-1]; for(i = n-1; i >= 0; i --) sa[-- wd[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 ++) wd[i] = 0; for(i = 0; i < n; i ++) wd[wv[i]] ++; for(i = 1; i < m; i ++) wd[i] += wd[i-1]; for(i = n-1; i >= 0; i --) sa[-- wd[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 ++; } }}void calHeight(int *r, 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 ++); }}int getId(int k) { int low = 0, high = n - 1, mid; while(low < high) { mid = (low+high) / 2; if(sum[mid] < k) { low = mid + 1; }else { high = mid; } } return low;}bool valid(int len, int out = 0){ int i = n + 1, j, k, id, cnt; bool f[nMAX]; while(1) { while(i <= m && height[i] < len) i ++; if(i > m) break; memset(f, 0, sizeof(f)); id = getId(sa[i-1]); f[id] = true; cnt = 1; while(i <= m && height[i] >= len) { id = getId(sa[i]); if(!f[id]) { f[id] = true; cnt ++; } i ++; } if(out == 0) { if(2 * cnt > n) return true; }else { if(2 * cnt > n) { for(k = sa[i-1], j = 0; j < len; k ++, j ++) { printf("%c", num[k] + 'a' - 100); } printf("n"); } } } return false;}int main(){ int i, j, k; while(scanf("%d", &n) && n != 0) { scanf("%s", str[0]); if(n == 1) { printf("%snn", str[0]); continue; } sum[0] = strlen(str[0]); for(i = 1; i < n; i ++) { scanf("%s", str[i]); sum[i] = sum[i-1] + strlen(str[i]) + 1; } for(k = i = 0; i < n; i ++) { for(j = 0; j < strlen(str[i]); j ++) { num[k ++] = str[i][j] - 'a' + 100; } num[k ++] = i; } m = k - 1; da(num, m + 1, 130); calHeight(num, m); int low = 0, high = m, mid; while(low < high) { mid = (low+high+1) / 2; if(valid(mid)) { low = mid; }else { high = mid - 1; } } if(low == 0) { printf("?nn"); }else { valid(low, 1); printf("n"); } } return 0;}
