Short Java solution with comments


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    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;
            //merge current node
            TreeNode t = new TreeNode(t1.val + t2.val);
            //merge right and left subtrees
            t.left = mergeTrees(t1.left, t2.left);
            t.right = mergeTrees(t1.right, t2.right);
            return t;
        }
    }
    

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