Valid Bracket sequence

-
 bool isValid(string s) 
    {
        stack<char>stk;
        int f=1,i;
        for(i=0;i<s.size() && f==1;i++)
        {
            if(stk.empty())
            {
                if((s[i]=='{' || s[i]=='[' || s[i]=='('))
                    stk.push(s[i]);
                else
                    f=0;
            }
            else if((s[i]=='{' || s[i]=='[' || s[i]=='('))
                stk.push(s[i]);
            else if(s[i]=='}')
            {
                if(stk.top()=='{')
                    stk.pop();
                else
                    f=0;
            }
            else if(s[i]==']')
            {
                if(stk.top()=='[')
                    stk.pop();
                else
                    f=0;
            }
            else if(s[i]==')')
            {
                if(stk.top()=='(')
                    stk.pop();
                else
                    f=0;
            }
        }
        if (f==1 && stk.empty())
            return true;
        return false;
    }

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