63. Unique Paths II Leetcode Java

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.

Solution:

This problem add a little twist to the previous one. 62. Unique Path

But clearly, it is not hard to know, that at the obstacle points, the possible paths would be 0, and nothing else need to be changed in the code.

Time complexity: O(m*n), Extra space: O(n)

 public int uniquePathsWithObstacles(int[][] obstacleGrid) {  
     if(obstacleGrid== null || obstacleGrid.length==0 || obstacleGrid[0].length==0) return 0;  
     int m=obstacleGrid.length;  
     int n=obstacleGrid[0].length;  
     int[] res= new int[n];  
     res[0]=1;  
     for(int i=0;i<m;i++){  
       for(int j=0;j<n;j++){  
         if(obstacleGrid[i][j]==1) res[j]=0;  
         else {  
           if(j!=0) res[j]+=res[j-1];  
         }  
       }  
     }  
     return res[n-1];  
   }