Yes, it's DP (left or right), and cache can speed it up.

Isn't this too easy to a 5kyu, or it's just easy for golang ?

This one is good, at first I thought my solution would time out, but actually it passed, which means O(n^2) for worst time complexity, can actually reach O(n * k) for the random batch test, where k is the average lengh of max Palindromic Substring from the test.

This basically gives the answer ...

Is this related to GCD and Extended Euclidean Algorithm ?

You need to choose n digits first, as the example shows, and build only from these chosen digits.
I've updated the final sentence to emphasize that.

Anyway let me add it.

I thought that's part of integer definition, except 0 itself, but other rules (end with 9) excluded that.

Fixed .. didn't create kata for a while, forget how it worked ..

The function has 3 params, 3 * 2 * 2 = 12, are all the cases ...

In mode 0, when owner already connected and clients >= N, it will be refused for more connections.

@Bind4Basics Because java code can pass [3, 100), but the same code translated to js and go can't, get timeout.
I'm not sure why.
Anyway I've changed Java back to [3, 100) and keep go and js [3, 25), the kata description is also changed accordingly to reflect that.

I edited the description, in the Tip section, added an item: For any invalid input, return false.

rename done, existing js solution may become invalid due to that, sorry for those users.

My bad I changed the description to [3, 25) , 25 excluded ..