589. N叉树的前序遍历

方法一：递归

/**
 * Definition for a Node.
 * type Node struct {
 *     Val int
 *     Children []*Node
 * }
 */
func preorder(root *Node) []int {
    if root == nil {
        return []int{}
    }
    //加入当前节点的值
    ret := []int{root.Val}
    //遍历所有子节点为根的树并加入结果中
    for i := 0; i < len(root.Children); i++ {
        ret = append(ret, preorder(root.Children[i])...)
    }
    return ret
}