#include<cstdio>

#include<cstring>

#include<algorithm>

using namespace std;

const int N = 110005;

int t1[N], t2[N], c[N];

void st(int *x, int *y, int *sa, int n, int m) {

	int i;

	for(i = 0; i < m; ++i) c[i] = 0;

	for(i = 0; i < n; ++i) ++c[x[y[i]]];

	for(i = 1; i < m; ++i) c[i] += c[i - 1];

	for(i = n - 1; i >= 0; --i) sa[--c[x[y[i]]]] = y[i];

}

void mkhz(int *a, int *sa, int n, int m) {

	int *t, *x = t1, *y = t2, i, j, p;

	for(i = 0; i < n; ++i) x[i] = a[i], y[i] = i;

	st(x, y, sa, n, m);

	for(p = 1, j = 1; p < n; j <<= 1, 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;

		st(x, y, sa, n, m);

		for(t = x, x = y, y = t, p = 1, x[sa[0]] = 0, i = 1; i < n; ++i)

			x[sa[i]] = y[sa[i]] == y[sa[i - 1]] && y[sa[i] + j] == y[sa[i - 1] + j] ? p - 1 : p++;

	}

}

void mkh(int *r, int *sa, int *rank, int *h, int n) {

	int i, j, k = 0;

	for(i = 1; i <= n; ++i) rank[sa[i]] = i;

	for(i = 1; i <= n; h[rank[i++]] = k)

		for(k ? --k : 0, j = sa[rank[i] - 1]; r[i + k] == r[j + k]; ++k);

}

bool vis[105];

char s[N];

int a[N], rank[N], sa[N], h[N], n, k, belong[105], sz, ci = 0, ans[N], ansnum;

bool can(int num) {

	memset(vis, 0, sizeof(vis));

	int cnt = 0, cont = 0;

	for(int i = 2; i <= n; ++i)

		if (h[i] >= num) {

			for(int j = 1; j <= k; ++j) {

				if (sa[i] > belong[j - 1] && sa[i] < belong[j])

					cnt += (int)(!vis[j]), vis[j] = 1;

				if (sa[i - 1] > belong[j - 1] && sa[i - 1] < belong[j])

					cnt += (int)(!vis[j]), vis[j] = 1;

			}

		} else {

			if (cnt > k / 2) ans[++cont] = sa[i - 1];

			cnt = 0;

			memset(vis, 0, sizeof(vis));

		}

	if (cnt > k / 2) ans[++cont] = sa[n - 1];

	if (cont != 0) ansnum = cont;

	return cont;

}

int main() {

	belong[0] = - 1;

	while (scanf("%d", &k), k) {

		n = sz = 1;

		ansnum = 0;

		for(int i = 1; i <= k; ++i) {

			scanf("%s", s + n);

			for(; s[n] != '\0'; ++n) a[n] = (int)s[n];

			a[n] = '#' + i;

			belong[sz++] = n;

			++n;

		}

		--n;

		mkhz(a, sa, n + 1, 400);

		mkh(a, sa, rank, h, n);

		int left = 1, right = n, mid;

		while (left <= right) {

			mid = (left + right) >> 1;

			if (can(mid)) left = mid + 1;

			else right = mid - 1;

		}

		if (ci) puts(""); ++ci;

		if (left == 1) puts("?");

		else {

			for(int i = 1; i <= ansnum; ++i) {

				for(int j = ans[i]; j < ans[i] + left - 1; ++j)

					putchar(s[j]);

				puts("");

			}

		}

	}

	return 0;

}