$30\mathrm{.}9\mathrm{.}2$: Use a recursive helper method to eliminate the need to extract substrings.

$597714\mathrm{.}2988516.$qx$3$zqy$7$

Use a recursive helper method to make clean$()$ more efficient, by eliminating the need to extract substrings.

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import java.util.Scanner;

$/**$

$*$ A recursive method that removes forbidden characters from a string.

$*/$

public class CleanUp

$\{$

public static String cleanHelper$($String str$,$ String forbidden, int index$)$

$\{$

$/*$ Your code goes here $*/$

if $($forbidden$.$contains$($current$))$

$\{$

$/*$ Your code goes here $*/$

$\}$

else

$\{$

$/*$ Your code goes here $*/$

$\}$