Merge Two BSTs into Array

1def getAllElements(root1: Optional[TreeNode], root2: Optional[TreeNode]) -> list[int]:
2    def inorder(root: Optional[TreeNode]) -> list[int]:
3        if not root:
4            return []
5        return inorder(root.left) + [root.val] + inorder(root.right)
6    a = inorder(root1)
7    b = inorder(root2)
8    i = j = 0
9    res = []
10    while i < len(a) and j < len(b):
11        if a[i] <= b[j]:
12            res.append(a[i]); i += 1
13        else:
14            res.append(b[j]); j += 1
15    res.extend(a[i:]); res.extend(b[j:])
16    return res