please help with the todo parts:

package tree;

import java.util.Optional;

public class TreeNode $\{$

private T value;

private Optional$>$ left;

private Optional$>$ right;

Optional$>$ left$()\{$ return Optional.empty$()$;$\}//$ TODO

Optional$>$ right$()\{$return Optional.empty$()$;$\}//$ TODO

T value$()\{$return null;$\}$

$//$ Code with NULL

$//$ private static String printInorder$($TreeNode node$)\{$

$//$ if $($node $==$ null$)\{$

$//$ return $""$;

$//\}$

$//$ return printInorder$($node$.$left$())+$ node.value$()+$ printInorder$($node$.$right$())$;

$//\}$

$//$ public String printInorder$()\{$

$//$ return printInorder$($this$)$;

public String printInorder$()\{$

return printInorder$($Optional$.$of$($this$))$;

$//$ could be Optional.ofNullable$($this$)$

$\}$

private static String printInorderNonNull$($TreeNode node$)\{$

return printInorder$($node$.$left$())+$ node.value$()+$ printInorder$($node$.$right$())$;

$\}$

static String printInorder$($Optional$>$ node$)\{$

return node.map$($TreeNode::printInorderNonNull$)$

$.$orElse$("")$;

$\}$

public boolean lookup$($T value$)\{$

$//$ TODO

return false;

$\}$

public String printPreorder$()\{$

$//$ TODO

return $""$;

$\}$

public String printPostorder$()\{$

$//$ TODO

return $""$;

$\}$

public int size$()\{$

return size$($root$)$;

$\}$

public int height$()\{$

$//$ TODO

return $0$;

$\}$

public void mirror$()\{$

$//$ TODO

$\}$

public void doubleTree$()\{$

$//$ TODO

$\}$

public boolean sameTree$($TreeNode other$)\{$

$//$ TODO

return false;

$\}$

$\}$

ppesae