Where Will the Ball Fall — LeetCode 1706 Python Solution

The problem

You have a 2-D grid of size m x n representing a box, and you have n balls. The box is open on the top and bottom sides.

Example

Input

grid = [[1,1,1,-1,-1],[1,1,1,-1,-1],[-1,-1,-1,1,1],[1,1,1,1,-1],[-1,-1,-1,-1,-1]]

Output

[1,-1,-1,-1,-1]

Explanation

This example is shown in the photo.

Python solution

class Solution:
    def findBall(self, grid: List[List[int]]) -> List[int]:
        def dfs(i: int, j: int) -> int:
            if i == m:
                return j
            if j == 0 and grid[i][j] == -1:
                return -1
            if j == n - 1 and grid[i][j] == 1:
                return -1
            if grid[i][j] == 1 and grid[i][j + 1] == -1:
                return -1
            if grid[i][j] == -1 and grid[i][j - 1] == 1:
                return -1
            return dfs(i + 1, j + 1) if grid[i][j] == 1 else dfs(i + 1, j - 1)

        m, n = len(grid), len(grid[0])
        return [dfs(0, j) for j in range(n)]

Complexity

Pattern: Matrix and Grid

Treat a 2-D grid as a graph whose neighbours are the four adjacent cells. LeetCode 1706. Where Will the Ball Fall is filed here on both counts: the reference solution below belongs to the algorithm family this hub collects, and LeetCode tags it Matrix.

The matrix and grid guide has the Python template for the pattern and the 216 LeetCode problems that use it.

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