The idea is simple, set all obstacles to be 0 while doing the DP. No extra space is used.

```
public class Solution {
public int uniquePathsWithObstacles(int[][] obstacleGrid) {
int m = obstacleGrid.length;
int n = obstacleGrid[0].length;
obstacleGrid[0][0]^=1;
for(int i = 1;i<m;i++){
obstacleGrid[i][0]=(obstacleGrid[i][0]==1)? 0:obstacleGrid[i-1][0];
}
for(int j = 1;j<n;j++){
obstacleGrid[0][j] =(obstacleGrid[0][j]==1)? 0: obstacleGrid[0][j-1];
}
for(int i = 1;i<m;i++){
for(int j =1;j<n;j++){
obstacleGrid[i][j] =(obstacleGrid[i][j]==1)? 0: obstacleGrid[i-1][j]+obstacleGrid[i][j-1];
}
}
return obstacleGrid[m-1][n-1];
}
}
```