[NOIP2002 提高组] 字串变换

#include <bits/stdc++.h>
using namespace std;
string a, b;
int n; // 规则数量
string A[10], B[10];
map<string, bool> vis;
map<string, int> ans;
queue<string> q;
int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    cin >> a >> b;
    n = 1;
    while (cin >> A[n] >> B[n])
        n++;
    n--;
    // bfs
    q.push(a);
    vis[a] = true;
    ans[a] = 0;
    ans[b] = -1;
    while (!q.empty() && ans[b] == -1)
    {
        string now = q.front();
        q.pop();
        if (ans[now] == 10)
            continue;
        // 第 i 个规则
        for (int i = 1; i <= n; i++)
        {
            // 第 j 个位置是否匹配规则
            for (int j = 0; j + A[i].size() - 1 < now.size(); j++)
            {
                bool flag = true;
                for (int k = j; k <= j + A[i].size() - 1; k++)
                    if (now[k] != A[i][k - j])
                    {
                        flag = false;
                        break;
                    }
                if (!flag)
                    continue;
                string temp = "";
                for (int k = 0; k < j; k++)
                    temp += now[k];
                temp += B[i];
                for (int k = j + A[i].size(); k < now.size(); k++)
                    temp += now[k];
                if (vis[temp] == false)
                {
                    ans[temp] = ans[now] + 1;
                    vis[temp] = true;
                    q.push(temp);
                }
            }
        }
    }
    if (ans[b] == -1)
        cout << "NO ANSWER!";
    else
        cout << ans[b];
    return 0;
}