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.

m and n will be at most 100.

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.

Solution 1. Recursion

 1 public class Solution {
 2     private int totalPaths = 0;
 3     public int uniquePathsWithObstacles(int[][] obstacleGrid) {
 4         if(obstacleGrid == null || obstacleGrid.length == 0 || obstacleGrid[0].length == 0){
 
 5             return 0;
 6         }
 7         findPaths(obstacleGrid, 0, 0);
 8         return totalPaths;
 9     }
10     private void findPaths(int[][] grid, int x, int y){
11         if(!isInBound(grid, x, y) || grid[x][y] == 1){
12             return;
13         }
14         if(x == grid.length - 1 && y == grid[0].length - 1){
 
15             totalPaths++;
16             return;
17         }
18         findPaths(grid, x + 1, y);
19         findPaths(grid, x, y + 1);
20     }
21     private boolean isInBound(int[][] grid, int x, int y){
22         return x >= 0 && x < grid.length && y >= 0 && y < grid[0].length;    
23     }
24 }

Solution 2. Dynamic Programming, O(m * n) runtime, O(m * n) space

 1 public class Solution {
 2     public int uniquePathsWithObstacles(int[][] obstacleGrid) {
 3         if(obstacleGrid == null || obstacleGrid.length == 0 || obstacleGrid[0].length == 0 || obstacleGrid[0][0] == 1) {
 4             return 0;
 5         }
 6         int m = obstacleGrid.length;
 7         int n = obstacleGrid[0].length;
 8         int[][] T = new int[m][n];
 9         int i = 0, j = 0;
10         for(; i < m; i++) {
11             if(obstacleGrid[i][0] == 1) {
12                 break;
13             }
14             T[i][0] = 1;
15         }
16         for(; j < n; j++) {
17             if(obstacleGrid[0][j] == 1) {
18                 break;
19             }
20             T[0][j] = 1;
21         }
22         for(i = 1; i < m; i++) {
23             for(j = 1; j < n; j++) {
24                 if(obstacleGrid[i][j] == 1) {
25                     T[i][j] = 0;
26                 }
27                 else {
28                     T[i][j] = T[i - 1][j] + T[i][j - 1];
29                 }
30             }
31         }
32         return T[m - 1][n - 1];
33     }
34 }

Solution 3. Dynamic Programming with space optimization, O(m * n) runtime, O(n) space

 1 public class Solution {
 2     public int uniquePathsWithObstacles(int[][] obstacleGrid) {
 3         if(obstacleGrid == null || obstacleGrid.length == 0 || obstacleGrid[0].length == 0){
 4             return 0;
 5         }
 6         int[][] T = new int[2][obstacleGrid[0].length];
 7         int i = 0;
 8         for(; i < obstacleGrid[0].length; i++){
 9             if(obstacleGrid[0][i] == 1){
10                 break;
11             }
12             T[0][i] = 1;
13         }
14         for(; i < obstacleGrid[0].length; i++){
15             T[0][i] = 0;
16         }
17         boolean reachable = (obstacleGrid[0][0] == 0 ? true : false);
18         for(int j = 1; j < obstacleGrid.length; j++){
19             if(reachable && obstacleGrid[j][0] == 0){
20                 T[j % 2][0] = 1;
21             }
22             else{
23                 reachable = false;
24                 T[j % 2][0] = 0;
25             }
26             for(int k = 1; k < obstacleGrid[0].length; k++){
27                 if(obstacleGrid[j][k] == 1){
28                     T[j % 2][k] = 0;
29                 }
30                 else{
31                     T[j % 2][k] = T[(j - 1) % 2][k] + T[j % 2][k - 1];
32                 }
33             }
34         }
35         return T[(obstacleGrid.length - 1) % 2][obstacleGrid[0].length - 1];
36     }
37 }

Related Problems

Unique Paths

Unique Paths III

[LintCode] Unique Paths II