[LeetCode]Word Ladder II

Given two words (start and end), and a dictionary, find all shortest transformation sequence(s) 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"]

Return

  [
    ["hit","hot","dot","dog","cog"],
    ["hit","hot","lot","log","cog"]
  ]

Note:

• All words have the same length.
• All words contain only lowercase alphabetic characters.

思考：参考http://blog.csdn.net/doc_sgl/article/details/13341405

class Solution {
public:
    vector<vector<string> > findLadders(string start, string end, unordered_set<string> &dict)
    {
        result_.clear();
        unordered_map<string, vector<string>> prevMap;
        for(auto iter = dict.begin(); iter != dict.end(); ++iter)
            prevMap[*iter] = vector<string>();
        vector<unordered_set<string>> candidates(2);
        int current = 0;
        int previous = 1;
        candidates[current].insert(start);
        while(true)
        {
            current = !current;
            previous = !previous;
            for (auto iter = candidates[previous].begin(); iter != candidates[previous].end(); ++iter)
                dict.erase(*iter);
            candidates[current].clear();
            
            for(auto iter = candidates[previous].begin(); iter != candidates[previous].end(); ++iter)
            {
                for(size_t pos = 0; pos < iter->size(); ++pos)
                {
                    string word = *iter;
                    for(int i = 'a'; i <= 'z'; ++i)
                    {
                        if(word[pos] == i)continue;
                        word[pos] = i;
                        if(dict.count(word) > 0)
                        {
                            prevMap[word].push_back(*iter);
                            candidates[current].insert(word);
                        }
                    }
                }
            }
            if (candidates[current].size() == 0)
                return result_;
            if (candidates[current].count(end)) break;
        }
        vector<string> path;
        GeneratePath(prevMap, path, end);
        return result_;
    }
    
private:
    void GeneratePath(unordered_map<string, vector<string>> &prevMap, vector<string>& path, const string& word)
    {
        if (prevMap[word].size() == 0)
        {
            path.push_back(word);
            vector<string> curPath = path;
            reverse(curPath.begin(), curPath.end());
            result_.push_back(curPath);
            path.pop_back();
            return;
        }
        path.push_back(word);
        for (auto iter = prevMap[word].begin(); iter != prevMap[word].end(); ++iter)
            GeneratePath(prevMap, path, *iter);
        path.pop_back();
    }
    vector<vector<string>> result_;
};