148_leetcode_Word Ladder II
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Given two words (start and end), and a dictionary, find all shortest transformation sequence(s) from start to end, such that:
- Only one letter can be changed at a time
- 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"] ]
1:注意特殊情况;2:确保中间单词在字典中,则开始喝结束单词可能不在字典中;3:使用广度优先遍历找到start到end是否存在,并保存每一个单词的前驱单词;4:在遍历的过程中使用unordered_set<string>保存当前单词在这一层中出现以及以前层单词的出现;5:使用DFS获得从start到end的路径。
vector<vector<string> > findLadders(string start, string end, unordered_set<string> &dict) { vector<vector<string> > result; if(start.length() == 0 || end.length() == 0 || start.length() != end.length() || start == end || dict.size() == 0) { return result; } unordered_map<string, vector<string> > myMap; unordered_set<string> curVector, nextVector; unordered_set<string> flag; bool found = false; curVector.insert(start); while(!curVector.empty() && !found) { unordered_set<string>::iterator itr = curVector.begin(); for(; itr != curVector.end(); itr++) { flag.insert(*itr); } itr = curVector.begin(); for(; itr != curVector.end(); itr++) { string tmpWord = *itr; for(int i = 0; i < (int)tmpWord.size(); i++) { for(char j = 'a'; j <= 'z'; j++) { if(tmpWord[i] == j) { continue; } string tmp = tmpWord; tmp[i] = j; if(tmp == end) { found = true; } if(tmp == end || (dict.find(tmp) != dict.end() && flag.find(tmp) == flag.end())) { nextVector.insert(tmp); myMap[tmp].push_back(tmpWord); } } } } curVector.clear(); swap(curVector, nextVector); } if(found) { vector<string> tmpVector; findLaddersCore(result, tmpVector, myMap, start, end); } return result; } void findLaddersCore(vector<vector<string> >& result, vector<string>& tmpVector, unordered_map<string, vector<string>>& myMap, string& start, string cur) { tmpVector.push_back(cur); if(cur == start) { result.push_back(tmpVector); reverse(result.back().begin(), result.back().end()); tmpVector.pop_back(); return; } vector<string> tmp = myMap[cur]; for(int i = 0; i < (int)tmp.size(); i++) { findLaddersCore(result, tmpVector, myMap, start, tmp[i]); } tmpVector.pop_back(); return; }
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