uva1375 難題

#include<iostream>
#include<algorithm>
#include<math.h>
#include<string.h>
#include<stdio.h>
#include<string>
#include<vector>
#include<queue>
#include<map>
#include<sstream>
#include<cassert>
using namespace std;
const int maxn = 50+5;
const int maxlen = 20 + 5;
const int maxs = 50 * 20 + 52 + 5;
int n, L,ns;
string rule[maxn];
string sym[maxs];
int car[maxs], cdr[maxs];
string dp[maxs][maxlen];

bool is_all_terminal(const string &s) {
	for (int i = 0; i < s.length(); i++) {
		if (!(s[i] >= 'a'&&s[i] <= 'z'))return false;
	}
	return true;
}
string min(const string&a, const string& b) {
	if (a == "-")return b;
	return a < b ? a : b;
}
struct Node {
	int x;
	string s;
	Node(int x,string s):x(x),s(s){}
	bool operator<(const Node& rhs) {
		return s > rhs.s;
	}
};
struct Transform {
	int target, empty;
	Transform(int t,int e):target(t),empty(e){}
};
vector<Transform>tr[maxs];
bool vis[maxs];
void search(int len) {
	memset(vis, 0, sizeof(vis));
	priority_queue<Node>q;
	for (int i = 0; i < ns; i++) {
		if (dp[i][len] != "-") {
			q.push(Node(i, dp[i][len]));
		}
	}
	while (!q.empty()) {
		Node u = q.top(); q.pop();
		int x = u.x;
		string s = u.s;
		if (vis[x])continue;
		vis[x] = true;
		for (int i = 0; i < tr[x].size(); i++) {
			int target = tr[x][i].target;
			int empty = tr[x][i].empty;
			if (dp[empty][0] == ""&&dp[target][len] == "-" || s < dp[target][len]) {
				dp[target][len] = s;
				q.push(Node(target, s));
			}
		}
	}
}
map<string, int>sym2id;
int ID(const string&s) {
	if (!sym2id.count(s)) {
		sym[ns] = s;
		sym2id[s] = ns++;
	}
	return sym2id[s];
}
void add_intermediate_symbol(const string& S) {
	int s = ID(S);
	if (S.length() < 2)return;
	int h = ID(S.substr(0, 1));
	int t = ID(S.substr(1, S.length() - 1));
	tr[h].push_back(Transform(s, t));
	tr[t].push_back(Transform(s, h));
	car[s] = h;
	cdr[s] = t;
}
int main() {
	while (cin >> n >> L && n) {
		sym2id.clear();
		ns = 0;
		ID("");
		for (int i = 0; i < maxs; i++)tr[i].clear();
		for (int i = 0; i < n; i++) {
			cin >> rule[i];
			int left = ID(rule[i].substr(0, 1));
			int right = ID(rule[i].substr(2));
			tr[right].push_back(Transform(left, ID("")));
			int len = rule[i].length();
			for (int j = 2; j < len; j++) {
				add_intermediate_symbol(rule[i].substr(j));
			}
		}
		for (int i = 0; i < ns; i++)
			for (int j = 0; j < L; j++)
				dp[i][j] = "-";
		for (int j = 0; j <= L; j++) {
			for (int i = 0; i < ns; i++) {
				if (sym[i].length() == j && is_all_terminal(sym[i]))dp[i][j] = sym[i];
				if (sym[i].length() < 2)continue;
				int s1 = car[i], s2 = cdr[i];
				for (int k = 1; k < j; k++) {
					if (dp[s1][k] != "-"&&dp[s2][j - k] != "-")
						dp[i][j] = min(dp[i][j], dp[s1][k] + dp[s2][j - k]);
				}
			}
			search(j);
		}
	}
	return 0;
}