算法提高之字串变换

• 核心思想：双向广搜

  • 双向bfs 建立两个队列 一起bfs到中间态
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•   #include <iostream>#include <cstring>#include <algorithm>#include <queue>#include <unordered_map>using namespace std;const int N = 6;int n;string A,B;string a[N],b[N];int extend(queue<string>& q, unordered_map<string, int>&da, unordered_map<string, int>& db, string a[N], string b[N]){int d = da[q.front()];  //确定当前层数while(q.size() && da[q.front()] == d)  //如果是同一层的就搜{auto t = q.front();q.pop();for(int i=0;i<n;i++)  //遍历所有规则{for(int j=0;j<t.size();j++){if(t.substr(j,a[i].size()) == a[i])  //有相应的子串{//构造新的字符串string r = t.substr(0,j) + b[i] + t.substr(j+a[i].size());if(db.count(r)) return da[t] + db[r] + 1;  //如果b中已经搜到过if(da.count(r)) continue;da[r] = da[t] + 1;q.push(r);}}}}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 = 0;  //记录步数 如果超过10 直接return -1while(qa.size() && qb.size()){int t;//优先扩展长度短的那个if(qa.size()<qb.size()) t = extend(qa,da,db,a,b);  //注意规则是a->belse t = extend(qb,db,da,b,a);  //规则是b->a 反向if(t <= 10) return t;  //没找到之前会返回一个>10的数if(++ step == 10) return -1;}return -1;}int main(){cin>>A>>B;while(cin>>a[n]>>b[n]) n++;int t = bfs();if (t == -1) puts("NO ANSWER!");else cout << t << endl;}