/*==========================================================*\| 后缀数组：| 这里num[0, n-1]为有效值,就是输入的字符串稍稍转化而成的数组| sa[1~n]为有效值,sa[i]=a则代表排在第i位的是第a个后缀。a属于[0~n-1]| rank[0~~n-1]是有效值 rank[i]=b则代表第 i 个后缀排在第b位,b属于[1~n]| height[2~~n]是有效值 height[i]=c 则代表排在第i位的后缀和排在第i-1的后缀的最长前缀长度是c。\*==========================================================*/const int Max = 200001;int 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];}//Doubling Algorithm, r为待匹配数组，n为总长度，m为字符范围void da(int *r, int n, int m){int i, j, p, *x = wa, *y = wb, *t;//wd是为了方便计算sa1的辅助数组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++; }//x存的rank，y变成废品了 }}void calHeight(int *r, int n) {//求height数组。 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++);;//printf("sdddddddddddddddddddddd\n"); }}int who[Max], yes[101], ii;int len, n, nn;int ans[Max], ss;int check(int mid){int i, j, k, t, s, flag = 0;for (i = 2; i <= len; i = j + 1) {for (; height[i] < mid && i <= len; i++) ;for (j = i; height[j] >= mid; j++) ;if (j - i + 1 < nn) continue;ii++; s = 0;for (k = i - 1; k < j; k++)if ((t = who[sa[k]]) != 0)if (yes[t] != ii)yes[t] = ii, s++;if (s >= nn) if (flag) ans[++ss] = sa[i - 1];else ans[ss = 1] = sa[i - 1], flag = 1;}return flag;}int r[Max];int main(){int i, j, k;char s[1005];int flag = 0;while (scanf("%d", &n) == 1 && n) {len = 0;for (i = 1; i <= n; ++i) {scanf("%s", s);int k = strlen(s);for (j = 0; j < k; ++j) {r[j + len] = s[j] + 100;who[j + len] = i;}r[len + k] = i;who[len + k] = 0;len += k + 1;}--len;r[len] = 0;memset(sa, 0, sizeof(sa));da(r, len + 1, 250);calHeight(r, len);height[len + 1] = -1;nn = n / 2 + 1;int min = 1, max = 1000;while (min <= max) {int mid = (min + max) >> 1;if (check(mid)) min = mid + 1;else max = mid - 1;}if (flag) printf("\n");else flag = 1;if (max == 0) printf("?\n");else {for (i = 1; i <= ss; ++i) {k = ans[i];for (j = 0; j < max; ++j)printf("%c", r[k + j] - 100);printf("\n");}}} return 0;}
