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#include <iostream>
#include <algorithm>
#include <cstring>
#include <set>
using namespace std;
const int N = 100110;
int n, T, Case;
int sa[N], rk[N], cnt[N], oldrk[N << 1], id[N], px[N];
string s;
int ht[N];
int num[N]; // 存每个下标对应的字符串标号
int maxv;
inline bool cmp(int x, int y, int k) {
return oldrk[x] == oldrk[y] && oldrk[x + k] == oldrk[y + k];
}
void SA() {
int i, p = 0, k, m = 300;
memset(cnt, 0, sizeof cnt);
for (i = 1; i <= n; i++) cnt[rk[i] = s[i]]++;
for (i = 1; i <= m; i++) cnt[i] += cnt[i - 1];
for (i = n; i >= 1; i--) sa[cnt[rk[i]]--] = i;
for (k = 1; k < n; k <<= 1, m = p) {
for (p = 0, i = n ; i > n - k; i--) id[++p] = i;
for (i = 1;i <= n; i++){
if (sa[i] > k) id[++p] = sa[i] - k;
}
memset(cnt, 0, sizeof cnt);
for (i = 1; i <= n; i++) cnt[px[i] = rk[id[i]]]++;
for (i = 1; i <= m; i++) cnt[i] += cnt[i - 1];
for (i = n; i >= 1; i--) sa[cnt[px[i]]--] = id[i];
memcpy(oldrk, rk, sizeof rk);
for (p = 0, i = 1; i <= n; i++){
rk[sa[i]] = cmp(sa[i], sa[i - 1], k) ? p : ++p;
}
}
for (int i = 1; i <= n; i++) {
if (rk[i] == 1) continue;
int k = max(ht[rk[i - 1]] - 1, 0);
while (i + k <= n && s[i + k]== s[sa[rk[i] - 1] + k]) k++;
ht[rk[i]] = k;
}
}
bool check(int len) {
set<int> S;
S.insert(num[sa[1]]);
for (int i = 2; i <= n; i++) {
while (ht[i] >= len && i <= n) {
// 对应的串编号加入S
S.insert(num[sa[i]]);
i++;
}
// 查看上面这段是否找到一组解
if (S.size() > T / 2) return true;
S.clear();
S.insert(num[sa[i]]);
}
return false;
}
void judge(int len) {
set<int> S;
bool flag = false;
S.insert(num[sa[1]]);
for (int i = 2; i <= n; i++) {
while (ht[i] >= len && i <= n) {
// 对应的串编号加入S
S.insert(num[sa[i]]);
i++;
}
// 查看上面这段是否找到一组解
if (S.size() > T / 2) {
for (int j = sa[i - 1]; j <= sa[i - 1] + len - 1; j++) putchar(s[j]);
puts("");
flag = true;
}
S.clear();
S.insert(num[sa[i]]);
}
if (!flag) puts("?");
}
void solve() {
SA();
// 二分长度
int l = 1, r = maxv;
while (l < r) {
int mid = l + r + 1 >> 1;
if (check(mid)) l = mid;
else r = mid - 1;
}
// cout << l << endl;
judge(l);
}
int main() {
while (scanf("%d", &T), T) {
if (Case) puts("");
memset(num, 0, sizeof num);
maxv = 1010;
s = "#";
string t;
if (T == 1) {
cin >> t;
s += t;
cout << t << endl;
continue;
}
int p = 0;
char c = 1;
for (int i = 0; i < T; i++) {
cin >> t;
maxv = min(maxv, (int)t.size());
s += t;
s += c;
c++;
if (c == 96) c = 123;
for (int j = p + 1; j < s.size(); j++) num[j] = i;
p += t.size() + 1;
}
n = s.size() - 1;
solve();
Case++;
}
return 0;
}
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