February 13, 2022-3-middle order traversal of binary tree

Given the root node of a binary tree root , Back to its Middle preface Traverse .

Example 1：

Input ：root = [1,null,2,3]
Output ：[1,3,2]
Example 2：

Input ：root = []
Output ：[]
Example 3：

Input ：root = [1]
Output ：[1]
Example 4：

Input ：root = [1,2]
Output ：[2,1]
Example 5：

Input ：root = [1,null,2]
Output ：[1,2]

Tips ：

The number of nodes in the tree is in the range [0, 100] Inside
-100 <= Node.val <= 100

java Code ：

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

    public void inorder(TreeNode root, List<Integer> res) {
        if (root == null) {
            return;
        }
        inorder(root.left, res);
        res.add(root.val);
        inorder(root.right, res);
    }
}