Leetcode 63: Unique Paths II

Follow up for "Unique Paths":

Now consider if some obstacles are added to the grids. How many unique paths would there be?

An obstacle and empty space is marked as 1 and 0 respectively in the grid.

For example,

There is one obstacle in the middle of a 3x3 grid as illustrated below.

[
  [0,0,0],
  [0,1,0],
  [0,0,0]
]

The total number of unique paths is 2.

Note: m and n will be at most 100.

 

 

 

 

public class Solution {
    public int UniquePathsWithObstacles(int[,] obstacleGrid) {
        int m = obstacleGrid.GetLength(0), n = obstacleGrid.GetLength(1);
        
        // optimization: we can do in-place
        var dp = new int[m, n];
        
        for (int i = m - 1; i >= 0; i--)
        {
            for (int j = n - 1; j >= 0; j--)
            {
                if (i == m - 1 && j == n - 1)
                {
                    if (obstacleGrid[i , j] == 1) return 0;
                    dp[i, j] = 1;
                }
                else if (i == m - 1)
                {
                    dp[i, j] = obstacleGrid[i , j] == 1 ? 0 : dp[i, j + 1];
                } 
                else if (j == n - 1)
                {
                    dp[i, j] = obstacleGrid[i , j] == 1 ? 0 : dp[i + 1, j];
                }
                else
                {
                    dp[i, j] = obstacleGrid[i , j] == 1 ? 0 : dp[i + 1, j] + dp[i, j + 1];
                }
            }
        }
        
        return dp[0, 0];
    }
}