val = $val;
$this->left = $left;
$this->right = $right;
}
}
/**
* @param TreeNode $root
* @return Integer[]
*/
function inorderTraversal($root) {
if($root === null){
return [];
}
$a[] = $root->val;
$b = inorderTraversal($root->left);
$c = inorderTraversal($root->right);
return array_merge($b, $a, $c);
}