Unique Paths II

You are given an `m x n` integer array `grid`. There is a robot initially located at the top-left corner (i.e., `grid[0][0]`). The robot tries to move to the **bottom-right corner** (i.e., `grid[m - 1][n - 1]`). The robot can only move either down or right at any point in time. An obstacle and space are marked as `1` or `0` respectively in `grid`. A path that the robot takes cannot include **any** square that is an obstacle. Return _the number of possible unique paths that the robot can take to reach the bottom-right corner_. The testcases are generated so that the answer will be less than or equal to `2 * 109`.   **Example 1:** **Input:** obstacleGrid = [[0,0,0],[0,1,0],[0,0,0]] **Output:** 2 **Explanation:** There is one obstacle in the middle of the 3x3 grid above. There are two ways to reach the bottom-right corner: 1. Right -> Right -> Down -> Down 2. Down -> Down -> Right -> Right ``` **Example 2:** **Input:** obstacleGrid = [[0,1],[0,0]] **Output:** 1 ```   **Constraints:** - `m == obstacleGrid.length` - `n == obstacleGrid[i].length` - `1 <= m, n <= 100` - `obstacleGrid[i][j]` is `0` or `1`.