LeetCode 20. Valid Parentheses

class Solution {
public:
    bool isValid(string s) {
        stack<char> a;
        int n=s.length();
        for (int i=0;i<n;i++){
            if (s[i]==‘(‘||s[i]==‘[‘||s[i]==‘{‘) a.push(s[i]);
            else {
                if (a.empty()) return false;
                char t=a.top();
                a.pop();
                if (s[i]==‘}‘&&t!=‘{‘) return false;
                if (s[i]==‘)‘&&t!=‘(‘) return false;
                if (s[i]==‘]‘&&t!=‘[‘) return false;  
            }
        }
        if (a.empty()) return true;
        else return false;
        
    }
};