Valid Parentheses

class Solution {public:    bool isValid(string s) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        stack<char> stk;        for(int i=0;i<s.length();++i){            if(s[i]=='('||s[i]=='['||s[i]=='{'){                stk.push(s[i]);            }else{                if(stk.empty())return false;                switch(stk.top()){                    case '(':                            if(s[i]!=')')return false;                            break;                    case '[':                            if(s[i]!=']')return false;                            break;                    case '{':                            if(s[i]!='}')return false;                            break;                                    }                stk.pop();            }        }        return stk.empty();    }};