BFS之单词最少转换次数

#include<iostream>
#include<set>
#include<queue>
#include<vector>
#include<string>
#include<map>
using namespace std;
bool connect(string &word1,string &word2)
{
	int cnt = 0;
	for(int i = 0; i < word1.length(); i++)
	if(word1[i] != word2[i])
	cnt++;
	return cnt == 1;
}
void construct_graph(string &beginword,vector<string> &wordlist, map<string ,vector<string> > &graph)
{
	wordlist.push_back(beginword);
	for(int i = 0; i < wordlist.size(); i++)
	graph[wordlist[i]] = vector<string>();
	
	for(int i = 0; i < wordlist.size(); i++)
		for(int j = i+1; j < wordlist.size(); j++)
	{
		if(connect(wordlist[i],wordlist[j]))
		{
			graph[wordlist[i]].push_back(wordlist[j]);
			graph[wordlist[j]].push_back(wordlist[i]);
		}
	 } 
}
int BFS_wordlist(string &beginword, string &endword, map<string, vector<string> > &graph)
{
	queue<pair<string,int> > q;
	set<string> visit;
	q.push(make_pair(beginword,1));
	visit.insert(beginword);
	while(!q.empty())
	{
		string node = q.front().first;
		int step = q.front().second;
		q.pop();
		if(node == endword)
		{
			return step;
		}
		
		vector<string> brothers = graph[node];
		for(int i = 0; i < brothers.size(); i++)
		{
			if(visit.find(brothers[i]) == visit.end())
			{
				q.push(make_pair(brothers[i], step+1));
				visit.insert(brothers[i]);
			}
		}
	}
	return 0;
}
int main()
{
	string beginword,endword;
	vector<string> wordlist;
	map<string ,vector<string> > graph;
	wordlist.push_back("hot");
	wordlist.push_back("dot");
	wordlist.push_back("dog");
	wordlist.push_back("lot");
	wordlist.push_back("log");
	wordlist.push_back("cog");
	cin>>beginword;
	cin>>endword;
	construct_graph(beginword,wordlist,graph);
	cout<<BFS_wordlist(beginword,endword,graph);
	return 0;
}