Either dynamic programming or memoisation, for which 5 sounds about right.

Without the performance requirement, it would be about a 6.

ETA: or did you actually mean "rating" instead of "ranking" ?

Thank you for the feedback, I will think of another unique katas

Yeah - I'm BORED - or I wouldn't be solving diamond string kata.

ahhh it's Johan agn (same as last time) ~~ XD

You too huh?

Again, kata had been retired when I had a complete solution (Ouchhh ~~ XDDDD)

• It's not specified how non-string inputs should be handled
• I'm pretty sure there's already a kata like this among numerous such "create triangle/diamond/cross/whatever pattern" tasks

UndefinedBehaviorSanitizer:DEADLYSIGNAL
==1==ERROR: UndefinedBehaviorSanitizer: SEGV on unknown address 0x000000000000 (pc 0x00000042c8ba bp 0x0000021f7100 sp 0x7ffd9b0c80a0 T1)
==1==The signal is caused by a READ memory access.
==1==Hint: address points to the zero page.
==1==WARNING: invalid path to external symbolizer!
==1==WARNING: Failed to use and restart external symbolizer!
#0 0x42c8b9 (/workspace/test+0x42c8b9)
#1 0x425597 (/workspace/test+0x425597)
#2 0x428213 (/workspace/test+0x428213)
#3 0x4269ee (/workspace/test+0x4269ee)
#4 0x426549 (/workspace/test+0x426549)
#5 0x42613b (/workspace/test+0x42613b)
#6 0x42c4f5 (/workspace/test+0x42c4f5)
#7 0x42585d (/workspace/test+0x42585d)
#8 0x7fea57989bf6 (/lib/x86_64-linux-gnu/libc.so.6+0x21bf6)
#9 0x404589 (/workspace/test+0x404589)

UndefinedBehaviorSanitizer can not provide additional info.
==1==ABORTING

I dont't understand why I have this error

No, but it won't work if the kata hasn't Node 12.x enabled. It's already asked and answered below.

Javascript flat() function is prohibited?

This deserves higher rating. Is this part of dynamic programming?

I can't unseee!!
ahh my eyes!!