Hi!

Nevermind, I forgot the null when returning, it was expected for the printer

I came up with a solution that's basically the same as you two:

var mergeTrees = function(t1, t2) {
if (!t1 && !t2) return null
if (!t1 || !t2) return t1 || t2
const newNode = new TreeNode(t1.val + t2.val)
newNode.left = mergeTrees(t1.left, t2.left)
newNode.right = mergeTrees(t1.right, t2.right)
return newNode
};