Given two words (beginWord and endWord), and a dictionary's word list, find all shortest transformation sequence(s) from beginWord toendWord, such that:

  1. Only one letter can be changed at a time
  2. Each intermediate word must exist in the word list

For example,

Given:
beginWord = "hit"
endWord = "cog"
wordList = ["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.
class Solution {
public:
vector<vector<string> > findLadders(string start, string end,
const unordered_set<string> &dict) {
unordered_set < string > visited; // 判重
unordered_map<string, vector<string> > father; // 树
unordered_set<string> current, next; // 当前层,下一层,用集合是为了去重
bool found = false;
current.insert(start);
visited.insert(start);
while (!current.empty() && !found) {
for (auto word : current){
visited.insert(word);
}
for (auto word : current) {
for (size_t i = ; i < word.size(); ++i) {
string new_word = word;
for (char c = 'a'; c <= 'z'; ++c) {
if (c == new_word[i])
continue;
swap(c, new_word[i]);
if (new_word == end)
found = true; //找到了
if (visited.count(new_word) ==
&& (dict.count(new_word) || new_word == end)) {
next.insert(new_word);
father[new_word].push_back(word);
}
swap(c, new_word[i]); // restore
}
}
}
current.clear();
swap(current, next);
}
vector < vector<string> > result;
if (found) {
vector < string > path;
buildPath(father, path, start, end, result);
}
return result;
}
private:
void buildPath(unordered_map<string, vector<string> > &father,
vector<string> &path, const string &start, const string &word,
vector<vector<string> > &result) {
path.push_back(word);
if (word == start) {
result.push_back(path);
reverse(result.back().begin(),result.back().end());
} else {
for (auto f : father[word]) {
buildPath(father, path, start, f, result);
path.pop_back();
}
}
}
};

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