DP.
public class Solution {
public int uniquePathsWithObstacles(int[][] grid) {
int m = grid.length;
int n = grid[0].length;
int[][] f = new int[m][n];
f[0][0] = grid[0][0] == 1 ? 0 : 1;
for (int i = 1; i < n; ++i) {
if (grid[0][i] != 1) f[0][i] = f[0][i - 1];
else f[0][i] = 0;
}
for (int i = 1; i < m; ++i) {
if (grid[i][0] != 1) f[i][0] = f[i - 1][0];
else f[i][0] = 0;
}
for (int i = 1; i < m; ++i) {
for (int j = 1; j < n; ++j) {
if (grid[i][j] != 1) f[i][j] = f[i - 1][j] + f[i][j - 1];
else f[i][j] = 0;
}
}
return f[m - 1][n - 1];
}
}