Given the root of a binary tree, return the preorder traversal of its nodes' values.
Example 1
1
\
2
/
3
- Input: root = [1, null, 2, 3]
- Output: [1, 2, 3]
Example 2
- Input: root = []
- Output: []
Example 3
- Input: root = [1]
- Output: [1]
Example 4
1
/
2
- Input: root = [1, 2]
- Output: [1, 2]
Example 5
1
\
2
- 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?