After this loop , stack contains unbalanced part of expression , i . e . , expression of the form " } } . . } { { . . { "
Stack < Character > s = new Stack < > ( ) ; for ( int i = 0 ; i < len ; i ++ ) { char c = expr . charAt ( i ) ; if ( c == ' } ' && ! s . empty ( ) ) { if ( s . peek ( ) == ' { ' ) s . pop ( ) ; else s . push ( c ) ; } else s . push ( c ) ; }