Solution.java

/**
 * 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 {
    private List<Integer>[] nums = new List[2];

    public boolean twoSumBSTs(TreeNode root1, TreeNode root2, int target) {
        Arrays.setAll(nums, k -> new ArrayList<>());
        dfs(root1, 0);
        dfs(root2, 1);
        int i = 0, j = nums[1].size() - 1;
        while (i < nums[0].size() && j >= 0) {
            int x = nums[0].get(i) + nums[1].get(j);
            if (x == target) {
                return true;
            }
            if (x < target) {
                ++i;
            } else {
                --j;
            }
        }
        return false;
    }

    private void dfs(TreeNode root, int i) {
        if (root == null) {
            return;
        }
        dfs(root.left, i);
        nums[i].add(root.val);
        dfs(root.right, i);
    }
}