94. Binary Tree Inorder Traversal

Given a binary tree, return the inorder traversal of its nodes' values.

For example:
Given binary tree [1,null,2,3],

   1
    
     2
    /
   3

return [1,3,2].

题目含义：用中序遍历树并输出

    private void middleParserTree(TreeNode root,List<Integer> result)
    {
        if (root == null) return;
       
        middleParserTree(root.left,result);
        result.add(root.val);
        middleParserTree(root.right,result);
    }
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        middleParserTree(root,result);
        return result;
    }