Connect K with Push-Up Drops

Jane StreetPhone screenMedium

Build a two-player connection game on a board that is the upper half of the integer grid: columns are any integers (negative allowed) and rows are 0, 1, 2, ... counting up from the bottom. PushConnect(k) starts with an empty board. The two players are the strings "B" and "R". A drop into a column places the new piece at row 0; if the column already holds pieces, every one of them moves up one row first. Each test drives one PushConnect(k) instance through a sequence of calls and checks every return value.

Part 1: Dropping pieces

Implement drop(player, col) and column(col).

drop(player, col) drops a piece for player into column col: the piece lands at row 0, and any pieces already in that column each move up one row first. It returns how many pieces that column holds after the drop.

column(col) returns the column's pieces as a string read from row 0 upward, so "RB" means R at row 0 and B at row 1. An empty column returns "".

For this part winners is not tested.

Example 1

init: [3]
operations: [["drop",["B",0]],["drop",["R",0]],["column",[0]],["drop",["B",5]],["column",[5]],["column",[2]],["drop",["B",0]],["column",[0]],["drop",["R",-1]],["column",[-1]]]
Output: [1,2,"RB",1,"B","",3,"BRB",1,"R"]

Why: Dropping R into column 0 pushes the lone B up, so column 0 reads RB from the bottom. A third drop gives BRB. Column 2 never received a piece, so it reads as an empty string.

Constraints

1 <= k <= 50
1 <= len(operations) <= 5000
-10^6 <= col <= 10^6
player is "B" or "R"