Leetcode13 - 20. Valid parentheses

class Solution {
    
    
public:
    bool isValid(string s) {
    
    
         stack<char> st;
         for(auto t:s){
    
       
                
             if(t=='('||t=='['||t=='{')st.push(t);
             else if((t==')'||t==']'||t=='}')&&st.empty())return false;//小心这种情况
             else if(t==')'&&st.top()=='(')st.pop();
             else if(t==']'&&st.top()=='[')st.pop();
             else if(t=='}'&&st.top()=='{')st.pop();
             else return false;
             
         }
         return st.empty();
    }
};