Leetcode Note: Go - Binary Tree Preorder Traversal

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func preorderTraversal(root *TreeNode) []int {
    traversal := []int{}

    var preorder func(*TreeNode)
    
    preorder = func(root *TreeNode) {
        if root == nil {
            return
        }
        
        traversal = append(traversal, root.Val)
        preorder(root.Left)
        preorder(root.Right)
    }
    
    preorder(root)

    return traversal
}