756. 金字塔转换矩阵 

 1 class Solution 
 2 {
 3     unordered_map<string,vector<char>> hash;
 4 public:
 5     bool pyramidTransition(string bottom, vector<string>& allowed) 
 6     {
 7         for(auto a : allowed) hash[a.substr(0,2)].push_back(a[2]);
 8         return DFS(bottom,"",0);
 9     }
10 
11     bool DFS(string& bottom,string upper,int site)
12     {
13         if(bottom.size() == 1) return true;
14         if(upper.size() + 1 == bottom.size()) return DFS(upper,"",0);
15         for(auto it: hash[bottom.substr(site,2)]) 
16         {
17             upper.push_back(it);
18             if(DFS(bottom,upper,site+1)) return true;
19             upper.pop_back();
20         }
21         return false;
22     }
23 };

 

posted @ 2020-04-20 14:39  Jinxiaobo0509  阅读(207)  评论(0)    收藏  举报