int ladderLength(string start, string end, unordered_set<string> &dict) {

// Using a map for two purposes:

// 1) store the distince so far.

// 2) remove the duplication

map<string, int> dis;

dis[start] = 1;

queue<string> q;

q.push(start);

while(!q.empty()){

string word = q.front();

q.pop();

if (word == end) {

break;

}

for (int i=0; i<word.size(); i++){

string temp = word;

for(char c='a'; c<='z'; c++){

temp[i] = c;

if (dict.count(temp)>0 && dis.count(temp)==0){

dis[temp] = dis[word] + 1;

q.push(temp);

}

}

}

}

return (dis.count(end)==0) ? 0 : dis[end];

}