100 IQ

r in subsequent calls of flatten are different "r's". See this video about recursion: What on Earth is Recursion? - Computerphile.

See
https://junli.netlify.app/en/overlapping-regular-expression-in-python/
For a bit of an explanation as to how the lookahead is working and a link to a relevant Stack Overflow question.

yes

Case 1:
EDCBA__..._____..__
__EDCBA..._____..__
___EDCAB.._____..__
____EDABC._____..__
_____EABCD_____..__
______ABCDE____..__
_______BCDE___A..__
_______.CDE___AB.__
_______..DE___ABC__
_______...E___ABCD_
_______...____ABCD_         E
_______..._____BCD_        AE
_______..._____.CD_       BAE
_______..._____..D_      CBAE
_______..._____..__     DCBAE


Case 2:
EDCBA__..._..______
__EDCBA..._..______
___EDCAB.._..______
____EDABC._..______
_____EABCD_..______
______ABCDE..______
_______BCDEA.______
_______.CDEAB______
_______..DEABC_____
_______...EABC_____          D
_______..._ABC_____         ED
_______..._.BC_____        AED
_______..._..C_____       BAED
_______..._..______      CBAED


Q: Are they the same?
A: No

Did you try it on paper?

errrr, well, I honestly don't remember... :/

I'll bet on some kind of heuristic, (it's the minimal possible value of a whatever the situation), but I cannot get how I found that, right now... x)

Thanks

The discs are inserted in the top of the board and drop down to the lowest available slot in that column just like the real game.

Please refer to the task :-)

The game consists of a grid (7 columns and 6 rows) and two players that take turns to drop their discs. The pieces fall straight down, occupying the next available space within the column. The objective of the game is to be the first to form a horizontal, vertical, or diagonal line of four of one's own discs.