[LeetCode] 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.

class Solution {
public:
    int ladderLength(string start, string end, unordered_set<string> &dict) {
        if(start.size() != end.size() || start.empty() || end.empty())  return 0;
        queue<string> path;
        path.push(start);
        int level = 1, count = 1;
        dict.erase(start);
        while(!dict.empty() && !path.empty())
        {
            string curword = path.front();
            path.pop();
            count--;
            for(int i = 0; i < curword.size(); i++)
            {
                string tmp = curword;
                for(char c = 'a'; c <= 'z'; c++)
                {
                    if(tmp[i] == c) continue;
                    tmp[i] = c;
                    if(tmp == end)  return level+1;
                    if(dict.find(tmp) != dict.end())
                    {
                        path.push(tmp);
                        dict.erase(tmp);
                    }
                }
            }
            if(count == 0)
            {
                count = path.size();
                level++;
            }
        }
        return 0;
    }
};

参考：http://blog.csdn.net/doc_sgl/article/details/13290909