[leetcode刷题系列]Word Ladder II

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;
}
};