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
}
