输入: [1,null,2,3]
   1
    \
     2
    /
   3
​
输出: [1,3,2]

class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        if(root != null){
            list.addAll(inorderTraversal(root.left));
            list.add(root.val);
            list.addAll(inorderTraversal(root.right));
        }
        return list;
    }
}

class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        LinkedList<TreeNode> stack = new LinkedList<>();
        while(!stack.isEmpty()||root!=null){
            while(root!=null){
                stack.push(root);
                root = root.left;
            }
            TreeNode node = stack.pop();
            list.add(node.val);
            if(node.right!=null){
                root = node.right;
            }
        }
        return list;
    }
}