leetcode126 Word Ladder II

思路：

宽搜过程中分层记录路径，递归还原。
实现：

class Solution 
{
public:
    void getPath(string now, string beginWord, string endWord, vector<string>& buf, unordered_map<string, unordered_set<string>>& par, vector<vector<string>>& ret)
    {
        if (now == beginWord)
        {
            vector<string> tmp(1, endWord);
            for (auto it : buf) tmp.push_back(it);
            ret.push_back(tmp);
            reverse(ret.back().begin(), ret.back().end());
            return;
        }
        for (auto it : par[now])
        {
            buf.push_back(it);
            getPath(it, beginWord, endWord, buf, par, ret);
            buf.pop_back();
        }
    }
    vector<vector<string>> findLadders(string beginWord, string endWord, vector<string>& wordList) 
    {
        unordered_set<string> tmp;
        for (auto it : wordList) tmp.insert(it);
        unordered_map<string, unordered_set<string>> par;
        unordered_set<string> q;
        q.insert(beginWord);
        bool flg = false;
        while (!q.empty())
        {
            unordered_set<string> next;
            for (auto it : q)
            {
                 for (int i = 0; i < it.length(); i++)
                {
                    for (char c = 'a'; c <= 'z'; c++)
                    {
                        string buf = it;
                        if (buf[i] == c) continue;
                        buf[i] = c;
                        if (!tmp.count(buf)) continue;
                        if (!q.count(buf)) 
                        {
                            next.insert(buf);
                            par[buf].insert(it);
                        }
                        if (buf == endWord) flg = true;
                    }    
                } 
            }
            for (auto it : q) { tmp.erase(it); }
            q = next;
            if (flg) break;
        }
        vector<vector<string>> ret;
        if (flg) 
        {
            vector<string> buf;
            getPath(endWord, beginWord, endWord, buf, par, ret);
        }
        return ret;
    }
};