If ' [ ' opening bracket , push it into character stack .
else if ( str [ i ] == ' [ ' ) { if ( str [ i - 1 ] >= '0' && str [ i - 1 ] <= '9' ) stringstack . push ( str [ i ] ) ; else { stringstack . push ( str [ i ] ) ; integerstack . push ( 1 ) ; } } else stringstack . push ( str [ i ] ) ; }