94. 二叉树的中序遍历

题目

给定一个二叉树的根节点 root ，返回 它的 中序 遍历 。

示例： 输入：root = [1,null,2,3] 输出：[1,3,2]

https://leetcode.cn/problems/binary-tree-inorder-traversal/

递归解法

/**
 * 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 LinkedList<>();
        if (root == null) {
            return res;
        }

        res.addAll(inorderTraversal(root.left));
        res.add(root.val);
        res.addAll(inorderTraversal(root.right));
        return res;
    }
}