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public TreeNode mergeTrees (TreeNode t1, TreeNode t2) {
        if(t1==null){
			return t2;
		}

		if(t2==null){
			return t1;
		}

		TreeNode head = new TreeNode(t1.val+t2.val);
		head.left = mergeTrees(t1.left, t2.left);
		head.right = mergeTrees(t1.right, t2.right);

		return head;

    }