原题出处：https://leetcode.cn/leetbook/read/top-interview-questions-medium/xv7pir/

解法一：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {

    private void inorder(TreeNode root,List<Integer> list) {
        if (root == null) {
            return;
        }
        // 遍历左节点
        inorder(root.left,list);
        // 访问节点
        list.add(root.val);
        // 遍历右节点
        inorder(root.right,list);
    }

    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        inorder(root,list);
        return list;
    }
}

思路：中序遍历，左、根、右。递归。