Leetcode 63: unique pathII

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.

跟unique path 的解法类似，同样可以维护一个一维数组，只是递推公式要考虑到有障碍的情况下，path = 0;

class Solution {
public:
    int uniquePathsWithObstacles(vector<vector<int>>& obstacleGrid) {
        //sanity check
        if (obstacleGrid.size() == 0 || obstacleGrid[0].size() == 0) return 0;
        vector<int> M(obstacleGrid[0].size(), 0);
         M[0] = 1;
        for (int i = 0; i < obstacleGrid.size(); i++) {
            for (int j = 0; j < obstacleGrid[0].size(); j++) {
                if (obstacleGrid[i][j] == 1) {
                    M[j] = 0;
                } else {
                   if (j > 0) M[j] += M[j-1];
                }
            }
        }
        return M[obstacleGrid[0].size()-1];
    }
};