二叉树前序遍历

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func preorderTraversal(root *TreeNode) []int {
    if root == nil {
        return []int{}
    }
    res := []int{root.Val}
    res = append(res, preorderTraversal(root.Left)...)
     res = append(res, preorderTraversal(root.Right)...)
     return res
}