我为什么错了第五点，手动演算也是没有问题的，为什么会错呢？求解释。

#include <bits/stdc++.h>
using namespace std;
string sts, eds, x[10], y[10];
int id;
bool flag = 0;
struct que{
    string s;
    int step;
};
map <string, string> tran;
map <string, bool> vis;
void bfs(string sts){
    queue <que> q;
    q.push({sts, 0});
    vis[sts] = 1;
    while(!q.empty()){
        string xx = q.front().s;
        int step = q.front().step; q.pop();
        if(xx == eds){
            cout << step << "\n";
            flag = 1;
            return ;
        }
        if(step == 10)
            continue;
        for(int i = 1; i <= id; i++){
            int j = 0;
            while(xx.find(x[i], j) != string::npos){
                int p = xx.find(x[i], j);
                string nx = xx.substr(0, p) + y[i] + xx.substr(p + x[i].size());
                if(vis[nx])
                    break;
                vis[nx] = 1;
                q.push({nx, step + 1});
                j = p + x[i].size();
            }
        }
    }
}

int main(){
    cin >> sts >> eds;
    while(cin >> x[++ id] >> y[id]){
        tran[x[id]] = y[id];
    }
    bfs(sts);
    if(!flag)
        cout << "NO ANSWER!" << "\n";
    return 0;
}