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