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.

Solution:

Same idea as Unique Paths.

The only difference is whenever we meet an obstacle, we set the number of unique paths to this point 0.



Code:

public class Solution {
    public int uniquePathsWithObstacles(int[][] obstacleGrid) {
        
        if (obstacleGrid == null || obstacleGrid.length == 0) {
            return 0;
        }
        if (obstacleGrid[0] == null || obstacleGrid[0].length == 0) {
            return 0;
        }
        
        int m = obstacleGrid.length;
        int n = obstacleGrid[0].length;
        int[][] paths = new int[m][n];
        
        for (int i = 0; i < m; i++) {
            if (obstacleGrid[i][0] == 1) {
                break;
            }
            paths[i][0] = 1;
        }
        for (int j = 0; j < n; j++) {
            if (obstacleGrid[0][j] == 1) {
                break;
            }
            paths[0][j] = 1;
        }
        
        for (int i = 1; i < m; i++) {
            for (int j = 1; j < n; j++) {
                if (obstacleGrid[i][j] == 1) {
                    paths[i][j] = 0;
                    continue;
                }
                paths[i][j] = paths[i - 1][j] + paths[i][j - 1];
            }
        }
        
        return paths[m - 1][n - 1];
    }
}