20. 有效的括号

 1 //思路很清晰，直接用stack
 2 class Solution 
 3 {
 4     unordered_map<char,char> hash = {{‘(‘,‘)‘},{‘[‘,‘]‘},{‘{‘,‘}‘}};
 5 public:
 6     bool isValid(string s) 
 7     {
 8         stack<char> stk;
 9         for(auto a : s)
10         {
11             if(a == ‘(‘ || a == ‘{‘ || a == ‘[‘) stk.push(a);
12             else
13             {
14                 if(!stk.empty() && a == hash[stk.top()]) stk.pop();
15                 else return false;
16             }
17         }
18         return stk.size() == 0;
19     }
20 };