Grid Unique Paths

Problem

Given a grid of size m x n, where each cell can either be an obstacle 1 or a free space 0, determine the number of unique paths from the top-left corner to the bottom-right corner. You can only move down or right at any step, and you cannot move through cells containing obstacles.

Examples

countPaths([
    [0, 0, 0],
    [1,  1, 0],
    [0, 0, 0]
])
// returns 1
// There are only one path: 
// - Right -> Right -> Down -> Down