63. 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.

int uniquePathsWithObstacles(vector<vector<int>>& o) {    int m = o.size();    int n = o[0].size();    vector<vector<int>> res(m, vector<int>(n, 1));    for (int i = 0; i < m; i++) {        for (int j = 0; j < n; j++) {            if (o[i][j])                res[i][j] = 0;            else if (i == 0 && j == 0)                res[i][j] = 1;            else {                int up = i == 0 ? 0 : res[i - 1][j];                int left = j == 0 ? 0 : res[i][j - 1];                res[i][j] = up + left;            }              }    }    return res[m - 1][n - 1];}