剑指 Offer II 114. 外星文字典

题面：

 题解：按照拓扑序即可。

代码：

class Solution {
public:
    string alienOrder(vector<string>& words) {
          int n = words.size();
          string ans="";
          unordered_map<char, unordered_set<int> >g;
          unordered_map<char, int> d;
          string s1, s2;
          int n1, n2;
          for(int i = 0;i < n;i ++)
          {
              for(auto t: words[i])
               d[t]=0;
          }
          for(int i = 1;i < n;i ++)
          {
              s1 = words[i - 1];
              s2 = words[i];
              n1 = s1.size();
              n2 = s2.size();
              int j;
              for(j = 0;j < min(n1, n2); j++)
              {
                  if(s1[j] != s2[j])
                  {
                      if(g[s1[j]].find(s2[j])==g[s1[j]].end())
                      {
                        d[s2[j]] ++;
                        g[s1[j]].insert(s2[j]);
                      }
                      break;
                  }
              }
              if(j == min(n1, n2)&&n1>n2)return "";
          }
          queue< char >q;
          for(auto t : d)
          {
              auto c = t.first;
              if(!d[c])
              {
                  q.push(c);
              }
          }
          while(q.size())
          {
              auto u = q.front();
              q.pop();
              ans += u;
              for(auto t: g[u])
              {
                  d[t]--;
                  if(!d[t])
                  {
                      q.push(t);
                  }
              }
          }
          //cout<<ans<<endl;
          if(ans.size() != d.size())return "";
          return ans;
    }
};