给定一个二叉树，返回它的中序 遍历。

示例:

输入: [1,null,2,3]

image.png

输出: [1,3,2]

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func inorderTraversal(root *TreeNode) []int {
    if root == nil{
        return []int{}
    }
    res := make([]int,0,10)
    var helper func(r *TreeNode)
    helper = func(r *TreeNode){
        if r == nil{
            return 
        }
        helper(r.Left)
        res = append(res,r.Val)
        helper(r.Right)
    }
    helper(root)
    return res
}