Binary Tree Preorder Traversal
You are given the root of a binary tree, return the preorder traversal of its nodes' values.
Example 1:
Input: root = [1,2,3,4,5,6,7]
Output: [1,2,4,5,3,6,7]Example 2:
Input: root = [1,2,3,null,4,5,null]
Output: [1,2,4,3,5]Example 3:
Input: root = []
Output: []Constraints:
0 <= number of nodes in the tree <= 100-100 <= Node.val <= 100
Follow up: Recursive solution is trivial, could you do it iteratively?