LeetCode - 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) {
        if(obstacleGrid==null) {
            return 0;
        }
        int m = obstacleGrid.length;
        if(m==0) {
            return 0;
        }
        int n = obstacleGrid[0].length;
        if(n==0) {
            return 0;
        }
        int step[][] = new int[m][n];
        for(int i=0;i<m;i++) {
            for(int j=0;j<n;j++) {
                int g = obstacleGrid[i][j];
                if(g==1) {
                    step[i][j] = 0;
                } else if(i==0 && j==0) {
                    step[i][j] = 1;
                } else if(i==0) {
                    step[i][j] = step[i][j-1];
                } else if(j==0) {
                    step[i][j] = step[i-1][j];
                } else {
                    step[i][j] = step[i-1][j] + step[i][j-1];
                }
            }
        }
        return step[m-1][n-1];
    }
}