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 {
    public List<Integer> getAllElements(TreeNode root1, TreeNode root2) {
        List<Integer> t1 = new ArrayList<>();
        List<Integer> t2 = new ArrayList<>();
        dfs(root1, t1);
        dfs(root2, t2);
        return merge(t1, t2);
    }

    private void dfs(TreeNode root, List<Integer> t) {
        if (root == null) {
            return;
        }
        dfs(root.left, t);
        t.add(root.val);
        dfs(root.right, t);
    }

    private List<Integer> merge(List<Integer> t1, List<Integer> t2) {
        List<Integer> ans = new ArrayList<>();
        int i = 0, j = 0;
        while (i < t1.size() && j < t2.size()) {
            if (t1.get(i) <= t2.get(j)) {
                ans.add(t1.get(i++));
            } else {
                ans.add(t2.get(j++));
            }
        }
        while (i < t1.size()) {
            ans.add(t1.get(i++));
        }
        while (j < t2.size()) {
            ans.add(t2.get(j++));
        }
        return ans;
    }
}