836. Preorder Traversal of Binary Tree

Given the root of a binary tree, return its preorder traversal (node, left subtree, right subtree) as an array of node values. The tree is given as a level-order array with null for missing children.

Input: A level-order array of the tree.

Output: Array — node values in preorder.

Examples

Input: [1,2,3,4,5,null,6]
Output: [1,2,4,5,3,6]
Explanation: Root, left, right order.

Input: [1]
Output: [1]
Explanation: Single node.

Input: []
Output: []
Explanation: Empty tree.

Constraints

• 0<=nodes<=10^4
• -1000<=value<=1000

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