1、B站视频链接：B19 双向BFS 字串变换_哔哩哔哩_bilibili

题目链接：[NOIP2002 提高组] 字串变换 - 洛谷

#include <iostream>
#include <queue>
#include <unordered_map>
using namespace std;const int N=7;
int n;
string A,B,a[N],b[N];int extend(queue<string>&q, unordered_map<string,int>&da,unordered_map<string,int>&db, string a[], string b[]){int m=q.size(); while(m--){                       //每次扩展一层auto f=q.front(); q.pop();      //父串出队for(int i=0; i<n; i++)          //枚举规则for(int j=0; j<f.size(); j++) //枚举父串中的位置if(f.substr(j,a[i].size())==a[i]){string s=f.substr(0,j)+b[i]+f.substr(j+a[i].size());if(da.count(s)) continue;if(db.count(s)) return da[f]+db[s]+1;da[s]=da[f]+1;  //变换步数q.push(s);      //子串入队}}return 11;
}
int bfs(){if(A==B) return 0;queue<string> qa,qb;unordered_map<string,int> da,db;qa.push(A),qb.push(B); da[A]=db[B]=0;int step=10, t;while(step--){if(qa.size()<=qb.size()) t=extend(qa,da,db,a,b);else t=extend(qb,db,da,b,a);if(t<=10) return t;}return 11;
}
int main(){cin>>A>>B;while(cin>>a[n]>>b[n]) n++;int t=bfs();t==11?printf("NO ANSWER!"):printf("%d",t);
}