All Elements in Two Binary Search Trees

/**
 * 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 {
    List<Integer> tree1 = new ArrayList<>();
    List<Integer> tree2 = new ArrayList<>();
    public List<Integer> getAllElements(TreeNode root1, TreeNode root2) {
        inorder(root1, tree1);
        inorder(root2, tree2);
        tree1.add(Integer.MAX_VALUE);
        tree2.add(Integer.MAX_VALUE);
        List<Integer> res = new ArrayList<>();
        int i = 0;
        int j = 0;
        while(res.size()!=tree1.size() + tree2.size()-2){
            if(tree1.get(i) < tree2.get(j)){
                res.add(tree1.get(i++));
            }
            else{
                res.add(tree2.get(j++));
            }
        }
        return res;
    }
    public void inorder(TreeNode root,  List<Integer> tofill){
        if(root == null) return;
        inorder(root.left, tofill);
        tofill.add(root.val);
        inorder(root.right, tofill);
        
    }
}