Given the `root`

of a binary tree, return *the inorder traversal of its nodes' values*.

**Example 1:**

**Input:** root = [1,null,2,3]
**Output:** [1,3,2]

**Example 2:**

**Input:** root = []
**Output:** []

**Example 3:**

**Input:** root = [1]
**Output:** [1]

**Example 4:**

**Input:** root = [1,2]
**Output:** [2,1]

**Example 5:**

**Input:** root = [1,null,2]
**Output:** [1,2]

**Constraints:**

- The number of nodes in the tree is in the range
`[0, 100]`

.
`-100 <= Node.val <= 100`

**Follow up:** Recursive solution is trivial, could you do it iteratively?

Given the `root`

of a binary tree, return *the inorder traversal of its nodes' values*.

**Example 1:**

**Input:** root = [1,null,2,3]
**Output:** [1,3,2]

**Example 2:**

**Input:** root = []
**Output:** []

**Example 3:**

**Input:** root = [1]
**Output:** [1]

**Example 4:**

**Input:** root = [1,2]
**Output:** [2,1]

**Example 5:**

**Input:** root = [1,null,2]
**Output:** [1,2]

**Constraints:**

- The number of nodes in the tree is in the range
`[0, 100]`

.
`-100 <= Node.val <= 100`

**Follow up:** Recursive solution is trivial, could you do it iteratively?