void buildPaths(const string& word, const string& beginWord,
const unordered_map<string, vector<string>>& parents,
vector<string>& path, vector<vector<string>>& result) {
if (word == beginWord) {
result.push_back(vector<string>(path.rbegin(), path.rend()));
return;
}
for (const string& p: parents.at(word)) {
path.push_back(p);
buildPaths(p, beginWord, parents, path, result);
path.pop_back();
}
}
void buildPaths(string &start, string &end, unordered_map<string, vector<string> > &m,
vector<string> &path, vector< vector<string> > &result) {
if (end == start) {
vector<string> clone(path);
clone.insert(clone.begin(), start);
result.push_back(clone);
return;
}
path.insert(path.begin(), end);
for (int i=0; i < m[end].size(); ++i) {
buildPaths(start, m[end][i], m, path, result);
}
path.erase(path.begin());
}
vector<vector<string>> findLadders(string beginWord, string endWord, vector<string>& wordList) {
unordered_set<string> dict(wordList.begin(), wordList.end());
if (!dict.count(endWord)) return {};
dict.erase(endWord);
queue<string> q;
q.push(beginWord);
unordered_map<string, vector<string>> parents;
unordered_map<string, int> levels;
levels[beginWord] = 1;
int level = 0;
bool found = false;
while (!q.empty() && !found) {
level ++;
for (int size = q.size(); size > 0; size--) {
string tmp = q.front();
q.pop();
for (int i=0; i < tmp.size(); i++) {
string temp = tmp;
for (char c='a'; c <= 'z'; c++) {
if (c == tmp[i])
continue;
temp[i] = c;
if (temp == endWord) {
parents[temp].push_back(tmp);
found = true;
} else if (levels.count(temp) && level < levels.at(temp)) {
parents[temp].push_back(tmp);
}
if (!dict.count(temp))
continue;
dict.erase(temp);
q.push(temp);
levels[temp] = levels.at(tmp) + 1;
parents[temp].push_back(tmp);
}
}
}
}
vector<vector<string>> result;
if (found) {
vector<string> path{endWord};
buildPaths(endWord, beginWord, parents, path, result);
}
return result;
}
vector< vector<string> > findLadders(string start, string end, unordered_set<string> &dict) {
vector<vector<string> > result;
queue<string> q;
q.push(start);
unordered_set<string> visited;
visited.insert(start);
unordered_map<string, vector<string> > m;
unordered_map<string, int> level;
level[start] = 1;
int maxLevel = -1;
while (!q.empty()) {
string tmp = q.front();
q.pop();
if (maxLevel != -1 && level[tmp] > maxLevel) {
break;
}
for (int i=0; i < tmp.size(); i++) {
for (char c='a'; c <= 'z'; c++) {
if (c == tmp[i]) continue;
string temp = tmp;
temp[i] = c;
if (temp == end) {
maxLevel = level[tmp];
if (m.find(end) == m.end()) {
m[end] = vector<string>();
}
m[end].push_back(tmp);
} else if (dict.find(temp) != dict.end()) {
if (visited.find(temp) == visited.end()) {
q.push(temp);
visited.insert(temp);
m[temp] = vector<string>();
m[temp].push_back(tmp);
level[temp] = level[tmp] + 1;
} else if (level[tmp]+1 == level[temp]) {
m[temp].push_back(tmp);
}
}
}
}
}
vector<string> path;
buildPaths(start, end, m, path, result);
return result;
}
