二叉树的后序遍历

func postorderTraversal(root *TreeNode) []int {
    res := []int{}
    var help func(root *TreeNode)
    help = func(root *TreeNode) {
        if root == nil {
            return
        }
        help(root.Left)
        help(root.Right)
        res = append(res, root.Val)
    }
    help(root)
    return res
}