詞梯遊戲

//無權最短路徑問題
#include <iostream>
#include <unordered_map>
#include <string>
#include <vector>
#include <queue>
using namespace std;

unordered_map<string,string>
findChain(const unordered_map<string, vector<string>> & adjacentWords,
          const string & first,const string & second)
{
    unordered_map<string, string> previousWord;
    queue<string> q;

    q.push(first);

    while(!q.empty())
    {
        string current=q.front();q.pop();
        auto itr=adjacentWords.find(current);

        const vector<string> & adj=itr->second;
        for(string  str:adj)
            if(previousWord[str]=="")
            {
                previousWord[str]=current;
                q.push(str);
            }
    }
    previousWord[first]="";

    return previousWord;
}

vector<string> getChainFromPreviousMap(
    const unordered_map<string, string> & previous,const string & second)
{
    vector<string> result;

    //去除previous的常量性，因爲const的map不能使用[]操作符
    auto & prev=const_cast<unordered_map<string, string> &>(previous);

    for(string current=second;current!="";current=prev[current])
        result.push_back(current);

    reverse(result.begin(), result.end());
    return result;
}


int main(int argc, const char * argv[]) {
    unordered_map<string, vector<string>> adj={{"zero",vector<string>{"hero"}},
        {"hero",vector<string>{"zero","here"}},{"here",vector<string>{"hire","hero"}},
        {"hire",vector<string>{"fire","here"}},{"fire",vector<string>{"hire","five"}},
        {"five",vector<string>{"fire"}}};

    auto previous=findChain(adj, "zero", "five");
    auto v=getChainFromPreviousMap(previous, "five");

    for(auto x:v)
    {
        cout<<x<<endl;
    }
    return 0;
}

結果
zero
hero
here
hire
fire
five