【双端广搜】字符串接龙

110. 字符串接龙

#include <iostream>
#include <cstring>
#include <algorithm>
#include <queue>
#include <unordered_map>

using namespace std;

const int N = 510;

int n;
string word[N];
// 如果两个队列共用st数组,那么两个队列永远不会碰头
// 因为在入队时我们会continue掉st为true的元素
// 而st为true时可能是另一个队列遍历掉的元素
unordered_map<string,bool> state, rstate;
unordered_map<string,int> d, rd;

int bfs(queue<string> &q, unordered_map<string,int> &dist,
    unordered_map<string,int> &rdist,
    unordered_map<string,bool> &st)
{
    auto cur = q.front();   q.pop();
    for(int i = 0; i < cur.size(); i ++ ) 
    {
        for(int j = 0; j < 26; j ++ )
        {
            string t = cur;       
            t[i] = 'a' + j;
            if(st[t]) 
            {
                q.push(t);
                dist[t] = dist[cur] + 1;
                st[t] = false;
                if(rdist[t])   return dist[t] + rdist[t] - 1;
            }
        }
    }
    return 0; // not found
}

int double_direction_bfs(string start, string goal)
{
    if(start == goal)   return 1;
    queue<string> q, rq;
    q.push(start);
    rq.push(goal);
    d[start] = 1;
    rd[goal] = 1;
    state[start] = false;
    rstate[goal] = false;
    // 每次只执行一个队列的bfs，优先执行队列中元素少的
    while(q.size() && rq.size())
    {
        int r;
        if(q.size() < rq.size())    r = bfs(q, d, rd, state);
        else r = bfs(rq, rd, d, rstate);
        if(r)   return r;
    }
    return 0;
}

int main()
{
    cin >> n;
    for(int i = 0; i < n + 2; i ++ )
    {
        cin >> word[i];
        state[word[i]] = true;
        rstate[word[i]] = true;
    }
    cout << double_direction_bfs(word[0], word[1]) << endl;
    return 0;
}