[Leetcode] Unique Paths II

Unique Paths II Mar 29 '12 4573 / 11497

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.

class Solution {
public:
    int uniquePathsWithObstacles(vector<vector<int> > &obstacleGrid) {
        auto &a = obstacleGrid;
        int n = a.size();
        if (n == 0) return 0;
        int m = a[0].size();
        vector<int> res(m, 0);
        if (a[0][0] == 0) res[0] = 1;
        for (int i = 1; i < m; i++) {
            if (res[i-1] > 0 && a[0][i] == 0) res[i] = res[i-1];
        }
        
        for (int i = 1; i < n; i++) {
            if (res[0] == 0 || a[i][0] == 1) res[0] = 0; 
            for (int j = 1; j < m; j++) {
                if (a[i][j] == 1) res[j] = 0;
                else res[j] += res[j-1];
            }
        }
        return res[m-1];
    }
};