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.

typedef std::pair<std::string,std::string> PAIR;void  getSolution(std::string &end, std::string& word, std::unordered_multimap<std::string, std::string> &map, std::vector<std::vector<std::string> > &vec, std::vector<std::string> &vec2) {if(word == end || map.find(word) == map.end()) {    if(word != end) return;    vec2.push_back(end);    vec.push_back(vec2);    vec2.pop_back();    return;}std::pair<std::unordered_map<std::string, std::string>::iterator, std::unordered_map<std::string, std::string>::iterator> ret;ret = map.equal_range(word);while(ret.first != ret.second) {    vec2.push_back(ret.first->first);    getSolution(end, ret.first->second, map, vec, vec2);    vec2.pop_back();    ret.first++;}}class Solution {public:std::vector<std::vector<std::string>> findLadders(std::string start, std::string end, std::unordered_set<std::string> &dict) {std::vector<std::vector<std::string>> vec;    //if(start == end) { vec.push_back(vector<string>(1, start)); return vec; }    std::unordered_multimap<std::string, std::string> edges;    std::unordered_set<std::string> previousNodes;    std::vector<std::unordered_set<std::string> > node_levels(2);    int curLevel = 0;    node_levels[curLevel].insert(end);    std::unordered_set<std::string>::iterator it;    while(!node_levels[curLevel].empty() && node_levels[curLevel].count(start) == 0) {        for(it = node_levels[curLevel].begin(); it != node_levels[curLevel].end(); ++it) {            for(size_t i = 0; i < it->size(); ++i) {                std::string node(*it);                for(node[i] = 'a'; node[i] <= 'z'; ++node[i]) {                    if(node == start) {                        node_levels[1-curLevel].insert(node);                        edges.insert(PAIR(start, *it));                        break;                    }                    if(previousNodes.count(node) || node_levels[curLevel].count(node) || dict.count(node) == 0)                         continue;                    node_levels[1-curLevel].insert(node);                    edges.insert(PAIR(node, *it));                }            }            previousNodes.insert(*it);        }        node_levels[curLevel].clear();        curLevel = !curLevel;    }    previousNodes.clear();    if(node_levels[curLevel].empty()) return vec;    std::vector<std::string> vec2;    getSolution(end, start, edges, vec, vec2);    return vec;}};