20. Valid Parentheses

//DAY 13 PROBLEM 3
//The approach is simple to handle different string characters seperately , all the opening braces when encountered are just pushed in the stack, but when we encounter
//the closing brackets, we check whether the top of the stack i.e. the last character of the string is same set of opening brace.
//Few edge cases are there to remember
class Solution {
public:
    bool isValid(string s) {
        if(s.length()==1)
            return false;
        bool flag=false;
        stack<char>st;
        
       for(int i=0;i<s.length();i++)
       {
           
           if(s[i]=='(')
           {
               st.push('(');
           }
         else  if(s[i]=='{')
         {
             st.push('{');
         }
           else if(s[i]=='[')
           {
               st.push('[');
           }
           else if(s[i]==')')
           {
           //🤦🏾 The top of the stack can't be accesed if the stack is empty, it will throw an error so we first check whether the stack is empty or not.Eg:'}]'
               if(!st.empty()&&st.top()=='(')
                   st.pop();
               else
               {
                   flag=true;
                   break;
               }
                   
           }
               else if(s[i]=='}')
           {
               if(!st.empty()&&st.top()=='{')
                   st.pop();
               else 
               {
                   flag=true;
                   break;
               }
                   
           }
               else if(s[i]==']')
           {
               if(!st.empty()&&st.top()=='[')
                   st.pop();
               else
               {
                   flag=true;
                   break;
               }
                   
           }
        }
        //😰Here along with checking for flag true, also check whether the stack is empty or not as we remove elements only when we encounter the closing brackets...
        //If the stack contains only the opening braces, then it can never be the valid parenthesis like '((' or '{['
        if(flag==true||!st.empty())
            return false;
        else
            return true;
    }
};