leetCode_1305.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> left = new ArrayList<>();
        List<Integer> right  = new ArrayList<>();
        
        inOrder(root1, left);
        inOrder(root2, left);
        
        Collections.sort(left);
        return left;
    }
    
    void inOrder(TreeNode root, List<Integer> list){
        if(root == null){
            return;
        }
        inOrder(root.left, list);
        list.add(root.val);
        inOrder(root.right, list);
        
    }
}