题解

#include<bits/stdc++.h>
using namespace std;
struct edge{
    string s;
    int step;
};

int main()
{
    int n;
    string a, b, x[30], y[30];
    queue<edge> q;
    set<string> st;
    cin >> a >> b;
    n = 0;
    while (cin >> x[++n] && cin >> y[n]);
    q.push({a, 0}); st.insert(a);
    while (!q.empty()){
        string now = q.front().s;
        int step = q.front().step;
        if (now == b){
            cout << step;
            return 0;
        }
        if (step == 10) continue;
        for (int i = 1; i <= n; i++){
            string::size_type found = -1;
            while (1){
                found = now.find(x[i], found+1);
                if (found == string::npos) break;
                string nxt = now;
                nxt.erase(found, x[i].size());
                nxt.insert(found, y[i]);
                if (st.find(nxt) != st.end()) continue;
                st.insert(nxt);
                q.push({nxt, step+1});
            }
        }
        q.pop();
    }
    cout << "NO ANSWER!";
    return 0;
}