Java solution super easy to read, no need to create extra nodes.

  • 0

    Use t1 to store the final tree. No need to create new node. No need to check both null case.

        public TreeNode mergeTrees(TreeNode t1, TreeNode t2) {
            if (t1 != null && t2 != null) {
                t1.val += t2.val;
                t1.left = mergeTrees(t1.left, t2.left);
                t1.right = mergeTrees(t1.right, t2.right);
            else if (t1 == null){
                t1 = t2;
            return t1;

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