字串变换

#include<bits/stdc++.h>
using namespace std;
const int N=200;
string a[N],b[N];
string _state,_end;
int n;
map<string,int>mp1,mp2;
int main()
{
    cin>>_state>>_end;
    while(cin>>a[n]>>b[n]) n++;
    mp1[_state]=1,mp2[_end]=1;
    queue<string>q1,q2;
    q1.push(_state);
    q2.push(_end);
    for(int step=1;step<=5;step++)
    {
        while(q1.size())
        {
            string top1=q1.front();
          
            q1.pop();
            for(int i=0;i<n;i++){
                for(int j=0;j<top1.length();j++)
                {
                    if(a[i] == top1.substr(j,a[i].length()))
                    {
                        string _next=top1;
                        _next.replace(j,a[i].length(),b[i]);
                        if(mp1[_next]) continue;
                        if(mp2[_next])
                        {
                            cout<<mp1[top1]-1+mp2[_next]<<endl;
                            return 0;
                        }
                        mp1[_next]=mp1[top1]+1;
                        q1.push(_next);
                    }
                }
            }
        }
        while(q2.size())
        {
            string top1=q2.front();
            q2.pop();
            for(int i=0;i<n;i++){
                for(int j=0;j<top1.length();j++)
                {
                    if(b[i] == top1.substr(j,b[i].length()))
                    {
                        string _next=top1;
                        _next.replace(j,b[i].length(),a[i]);
                        if(mp2[_next]) continue;
                        if(mp1[_next])
                        {
                            cout<<mp2[top1]-1+mp1[_next]<<endl;
                            return 0;
                        }
                        mp2[_next]=mp2[top1]+1;
                        q2.push(_next);
                    }
                }
            }
        }
    }    
    puts("NO ANSWER!");
    return 0;
}