94. 二叉树的中序遍历（递归）
/**
* 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 ListinorderTraversal(TreeNode root) { List res = new ArrayList (); inorder(root, res); return res; } public void inorder(TreeNode root, List res){ //结点为空的情况 if(root == null){ return; } //递归 inorder(root.left, res); res.add(root.val); inorder(root.right, res); } }