BM23 二叉树的前序遍历

https://www.nowcoder.com/practice/5e2135f4d2b14eb8a5b06fab4c938635

func preorderTraversal( root *TreeNode ) []int {
    arr := make([]int, 0)
    preorder(root, &arr)
    return arr
}

func preorder(root *TreeNode, arr *[]int) {
    if root == nil {
        return
    }
    *arr = append(*arr, root.Val)
    preorder(root.Left, arr)
    preorder(root.Right, arr)
}