leetcode63.[DP] 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]
]

class Solution(object):    def uniquePathsWithObstacles(self, obstacleGrid):        m=len(obstacleGrid)        n=len(obstacleGrid[0])        num=[]        for i in range(m):            num.append([])            for j in range(n):                if obstacleGrid[i][j]==1:                    num[i].append(0)                elif i==0 and j==0:                    num[i].append(1)                elif i==0:                    num[i].append(num[i][j-1])                elif j==0:                    num[i].append(num[i-1][j])                else:                    num[i].append(num[i][j-1]+num[i-1][j])        return num[m-1][n-1]