LeetCode 训练场：94. 二叉树的中序遍历

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

public void inOrder(TreeNode root, List<Integer> list){
    if(root != null){
        inOrder(root.left, list);
        list.add(root.val);
        inOrder(root.right, list);
    } 
}