Flow Problem
Time limit2sMemory limit2048 MB
Given a 2 x n grid with a cyclic flow of unknown direction, find a sequence of moves that drops a token from the left and from the right within the move limit.
- Level
Medium7 of 10
- Topics
- Simulation, Implementation, Game theory
- Solved
- No attempts yet
Problem
This is an interactive problem.
There is a grid (), where rows are numbered from to , and columns are numbered from to . Each cell of the grid has flow in one of the four directions: up, down, left, or right. The flow directions in the grid form a continuous cycle, either clockwise or counterclockwise. The figure below shows a grid with clockwise flow.

You are playing a two-round game. At the start of each round, a token is placed on some cell. You make moves, and before every move, you know the token's current position. The goal is to drop one token from the left side of the grid and drop another token from the right side. It does not matter which side is used in which round. The size of the grid and the direction of the flow are unknown, but they are fixed in advance and do not change between rounds.
You can make the following moves.
- Move the token to an adjacent cell to the left, right, or in the other row. If there is no such cell, the token falls from the grid.
- Wait for seconds (). Each second, the token moves to the adjacent cell in the direction of the flow in its current cell.
Before each move, you are given the current coordinates of the token. The number of moves you can use is limited. See the details in the next section.