我的代码WA3个点。

调了2天了，救救孩子吧！

#include <bits/stdc++.h>
using namespace std;

typedef long long ll;
typedef pair<int, int> pii;

void fastio(const char *infile, const char *outfile) {
	if (infile) freopen(infile, "r", stdin);
	if (outfile) freopen(outfile, "w", stdout);
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	cout.tie(nullptr);
}

int n, m;
string s[10];
string t[100010];

void solve(string now, int stp) {
	now += s[stp];
	if (stp == n) {
		if (now.size() >= 3 && !binary_search(t + 1, t + 1 + m, now)) {
			cout << now << endl;
			exit(0);
		}
	} else for (int i = 1; now.size() + i + s[stp + 1].size() <= 16; i++) {
		now += "_";
		solve(now, stp + 1);		
	}
}

int main() {
	fastio(nullptr, nullptr);
	cin >> n >> m;
	for (int i = 1; i <= n; i++) cin >> s[i];
	for (int i = 1; i <= m; i++) cin >> t[i];
	sort(s + 1, s + 1 + n);
	sort(t + 1, t + 1 + m);
	do solve("", 1);
	while (next_permutation(s + 1, s + 1 + n));
	cout << -1 << endl;
	return 0;
}