[LeetCode] #63 Unique Paths II

public class Solution {
    // f[i][j] = how many paths from (0,0) to (i,j)
    // f[i][j] = f[i - 1][j] + f[i][j - 1]
    public int uniquePathsWithObstacles(int[][] obstacleGrid) {
        if (obstacleGrid == null || obstacleGrid.length == 0 || obstacleGrid[0].length == 0) {
            return 0;
        }
        int m = obstacleGrid.length;
        int n = obstacleGrid[0].length;
        int[][] f = new int[m][n];
        for (int i = 0; i < m; i++) {
            if (obstacleGrid[i][0] == 1 || (i > 0 && f[i - 1][0] == 0)) {
                f[i][0] = 0;
            } else {
                f[i][0] = 1;
            }
        }
        for (int i = 0; i < n; i++) {
            if (obstacleGrid[0][i] == 1 || (i > 0 && f[0][i - 1] == 0)) {
                f[0][i] = 0;
            } else {
                f[0][i] = 1;
            }
        }
        for (int i = 1; i < m; i++) {
            for (int j = 1; j < n; j++) {
                if (obstacleGrid[i][j] == 1) {
                    f[i][j] = 0;
                } else {
                    f[i][j] = f[i - 1][j] + f[i][j - 1];
                }
            }
        }
        return f[m - 1][n - 1];
    }
}