[leetcode刷题系列]Word Ladder II

这题时间卡的还真是严啊- - 

先bfs一遍，标记没个单词属于哪一层， 然后从end开始往回走，就好了

string start, end;unordered_map<string, int> lev;void dfs(string now, vector<string> & stk, vector<vector<string> > & ret){    if(now == start){        vector<string> tmp(stk);        tmp.push_back(now);        reverse(tmp.begin(), tmp.end());        ret.push_back(tmp);        return ;    }    stk.push_back(now);    for(int i = 0; i < now.size(); ++ i)        for(char c = 'a'; c <= 'z'; ++ c){            string next(now);            next[i] = c;            if(next == now)                continue;            if(lev.find(next) != lev.end() && lev[next] == lev[now] - 1){                dfs(next, stk, ret);            }        }    stk.pop_back();}class Solution {public:    vector<vector<string>> findLadders(string start, string end, unordered_set<string> &dict) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        vector<vector<string> > ret;        if(start == end){            ret.resize(1);            ret[0].push_back(start);            return ret;        }        ::start = start;        ::end = end;        lev.clear();        queue<string> q;        lev[start] = 0;        q.push(start);        while(!q.empty()){            string now = q.front();            q.pop();            if(now == end)                continue;            for(int i = 0; i < now.size(); ++ i)                for(char c = 'a'; c <= 'z'; ++ c){                    string next(now);                    next[i] = c;                    if(next == end || dict.find(next) != dict.end())                        if(lev.find(next) == lev.end()){                            lev[next] = lev[now] + 1;                            q.push(next);                        }                }        }        vector<string> stk;        dfs(end, stk, ret);        return ret;    }};