【LeetCode】94. 二叉树的中序遍历

给定一个二叉树的根节点 root ，返回它的 中序 遍历。

输入：root = [1,null,2,3]
输出：[1,3,2]

输入：root = []
输出：[]

输入：root = [1]
输出：[1]

输入：root = [1,2]
输出：[2,1]

输入：root = [1,null,2]
输出：[1,2]

/**
 * 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> result = new ArrayList<>();
        infixOrder(root, result);
        return result;
    }
    public void infixOrder(TreeNode root, List<Integer> result) {
        if (root == null) {
            return;
        }
        // 左子树递归
        infixOrder(root.left, result);
        result.add(root.val);
        // 右子树递归
        infixOrder(root.right, result);
    }
}