A robot starts at the top-left of an `m x n` grid and wants to reach the bottom-right, moving only right or down. Some cells contain obstacles (marked `1`); the robot cannot enter them.
Return the number of unique paths. If the start or end is blocked, the answer is `0`.
Example 1
Input:obstacleGrid = [[0,0,0],[0,1,0],[0,0,0]]
Output:2
Explanation:Two paths route around the center obstacle.
Constraints
- m == obstacleGrid.length
- n == obstacleGrid[i].length
- 1 <= m, n <= 100
- obstacleGrid[i][j] is 0 or 1