/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public TreeNode mergeTrees(TreeNode t1, TreeNode t2) {
        
        if(t1==null && t2== null) return null;
        if(t1 == null) return t2;
        if(t2 == null) return t1;
                
        TreeNode t3 = new TreeNode(t1.val + t2.val);
        t3.left = mergeTrees(t1.left, t2.left);
        t3.right = mergeTrees(t1.right, t2.right);
        
        return t3;
        
    }
}