Main.java:16: error: incompatible types: String cannot be converted to List<String>
        	ans.add(cur.toString());
        	                    ^
Main.java:32: error: incompatible types: List<String> cannot be converted to List<List<String>>
    	cb(ans,cur,n,0,0);
    	   ^
Note: Some messages have been simplified; recompile with -Xdiags:verbose to get full output
2 errors