LeetCode Problem : Unique Paths II

Problem

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.

Code

int uniquePathsWithObstacles(vector<vector<int> > &obstacleGrid) {
    // IMPORTANT: Please reset any member data you declared, as
    // the same Solution instance will be reused for each test case.
    int m = obstacleGrid.size();
    int n = obstacleGrid[0].size();
    vector<vector<int> > M(m,vector<int>(n,0));
    if(obstacleGrid[0][0] == 0)
        M[0][0] = 1;
    for(int i = 1;i < m;++i){
        if(obstacleGrid[i][0] == 0)
            M[i][0] = M[i - 1][0];
    }
    for(int j = 1;j < n;++j){
        if(obstacleGrid[0][j] == 0)
            M[0][j] = M[0][j - 1];
    }
    for(int i = 1;i < m;++i){
        for(int j = 1;j < n;++j){
            if(obstacleGrid[i][j] == 0)
            M[i][j] = M[i - 1][j] + M[i][j - 1];
        }
    }
    return M[m - 1][n - 1];
}