[数据结构与算法]AcWing 190. 字串变换（双向广搜）

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Code

const int N = 5e5 + 3;
int n;
string s, ss, a[10], b[10];
queue<string> q, qq;
unordered_map<string, int> ma, mb;

int extend(queue<string>& q, unordered_map<string, int>& ma,
	unordered_map<string, int>& mb, string a[], string b[])
{
	string x = q.front(); q.pop();

	for(int i = 0; i < n; i++)
		for(int j = 0; j < sz(x); j++)
			if (x.substr(j, sz(a[i])) == a[i])
			{
			    assert(j + sz(a[i])<= sz(x);
				string y = x.substr(0, j) + b[i] + x.substr(j + sz(a[i]));
				if (mb.count(y)) return ma[x] + mb[y] + 1;
				if (ma.count(y)) continue;
				ma[y] = ma[x] + 1;
				q.push(y);
			 }

	return 11;
}

int bfs()
{
	if (s == ss) return 0;

	q.push(s), qq.push(ss);
	ma[s] = mb[ss] = 0;

	while (sz(q) && sz(qq))
	{
		int t;
		if (sz(q) < sz(qq)) t = extend(q, ma, mb, a, b);
		else t = extend(qq, mb, ma, b, a);

		if (t <= 10) return t;
	}

	return -1;
}

int main()
{
	cin >> s >> ss;
	while (cin >> a[n] >> b[n])
		n++;

	int t = bfs();
	if (t == -1) cout << "NO ANSWER!" << endl;
	else cout << t << endl;


	return 0;
}

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