Decode String

//https://leetcode.com/problems/decode-string/


class Solution {
public:
    string decodeString(string s) {
        
        
        string ans ; 
        stack < int > stk1 ; 
        stack < char > stk2 ; 
        int n = s.length(), num = 0  ;
        for ( int i = 0 ; i < n ; i ++ )
        {
            if (  ( s [ i ] >= 'a' and s [ i ] <= 'z' ) or( s [ i ] >= 'A' and s [ i ] <= 'Z' )  )
                    stk2.push ( s [ i ] ) ; 
            else if (  s[ i ] >= '0' and s [ i ] <= '9' )
                    num = num * 10 + int ( s [ i ] - '0' ) ; 
            else if ( s [ i ] == '[' )
            {
                stk2.push ( '[' ) ; 
                stk1.push ( num ) ; 
                num = 0 ; 
            }
            else 
            {
                string temp, ss ; 
                while ( !stk2.empty() and stk2.top() != '[' )
                {
                    temp += stk2.top() ; 
                    stk2.pop() ; 
                }
                stk2.pop() ;
                int c = stk1.top() ; 
                stk1.pop() ; 
                reverse ( temp.begin() , temp.end() ) ; 
                while ( c --)
                        ss += temp ; 
                
                for ( int  j = 0 ; j < ss.length() ; j ++ )
                        stk2.push ( ss [ j ] ) ; 
                
            }
        }
        
        while ( !stk2.empty() )
        {
                ans += stk2.top() ; 
                stk2.pop() ; 
        }
        //cout << ans << endl ; 
        reverse ( ans.begin() , ans.end() ) ; 
        return ans ; 
    }
};