Word Ladder

Given two words (start and end), and a dictionary, find the length of shortest transformation sequence from start to end, such that:

1. Only one letter can be changed at a time
2. Each intermediate word must exist in the dictionary

For example,

Given:
start = "hit"
end = "cog"
dict = ["hot","dot","dog","lot","log"]

As one shortest transformation is "hit" -> "hot" -> "dot" -> "dog" -> "cog",
return its length 5.

Note:

• Return 0 if there is no such transformation sequence.
• All words have the same length.
• All words contain only lowercase alphabetic characters

思路：这道题使用BFS(广度优先搜索),使用队列作为辅助空间，将遍历得到满足条件的字符串放入path中，并且遍历到的字符串在dict出现过，我们也要删除dict中该字符串，防止以后再遍历到此字符串。

class Solution {

public:

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

        if(start.size()!=end.size())

            return 0;

        if(start.empty()||end.empty())

            return 0;

        if(dict.size()==0)

            return 0;

        queue<string> path;

        path.push(start);

        int level=1;

        int count=1;

        dict.erase(start);

        while(dict.size()>0 && !path.empty())

        {

            string s=path.front();

            path.pop();

            count--;

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

            {

                string temp(s);

                for(char ch='a';ch<='z';ch++)

                {

                    if(temp[i]==ch)

                        continue;

                    temp[i]=ch;

                    if(temp==end)

                        return level+1;

                    if(dict.find(temp)!=dict.end())

                    {

                        path.push(temp);

                    }

                    dict.erase(temp);

                }

            }

            if(count==0)

            {

                count=path.size();

                level++;

            }

        }

        return 0;

    }

};