代码：

#include <bits/stdc++.h>
using namespace std;
string a, b, a1[10], b1[10];
int l;
struct Person {
    int d;
    string s;
};
queue<Person> q;
unordered_map<string, int> mp;
void solve() {
    q.push((Person){ 0, a });
    while (!q.empty()) {
        Person v = q.front();
        q.pop();
        string s = v.s;
        int d = v.d;
        if(mp[s]) {
        	continue;
		}
        if (s == b && d <= 10) {
            cout << d;
            exit(0);
        }
        if (d > 10) {
            cout << "NO ANSWER!\n";
            exit(0);
        }
        mp[s] = 1;
        for (int i = 1; i <= l; ++i) {
            int num = s.find(a1[i], 0);
            while (num != -1) {
                string s1 = s;
                s1.replace(num, a1[i].size(), b1[i]);
                q.push((Person){ d + 1, s1 });
                num = s1.find(a1[i], num + 1);
            }
        }
    }
}
int main() {
    cin >> a >> b;
    int tmp = 1;
    while (cin >> a1[tmp] >> b1[tmp++])
        ;
    l = tmp - 1;
    solve();
    cout << "NO ANSWER!";
    return 0;
}

报replace越界了，萌新求问。