[leetcode] 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.

https://oj.leetcode.com/problems/unique-paths-ii/

 

思路：同Unique Paths一样，也是DP，障碍物特殊处理：

1. 初始化时，第一行和第一列，如果有障碍，则后面都不可达。
2. 计算其他格子时，有障碍的情况为0即可。

 

public class Solution {

    public int uniquePathsWithObstacles(int[][] obstacleGrid) {
        if (obstacleGrid == null || obstacleGrid.length == 0)
            return 0;
        int m = obstacleGrid.length;
        int n = obstacleGrid[0].length;

        int[][] step = new int[m][n];

        int i, j;
        for (i = 0; i < m; i++) {
            if (obstacleGrid[i][0] == 1)
                break;
            else
                step[i][0] = 1;

        }
        for (i = 0; i < n; i++) {
            if (obstacleGrid[0][i] == 1)
                break;
            else
                step[0][i] = 1;
        }

        for (i = 1; i < m; i++)
            for (j = 1; j < n; j++) {
                if (obstacleGrid[i][j] == 0) {
                    step[i][j] = step[i - 1][j] + step[i][j - 1];
                }

            }

        return step[m - 1][n - 1];
    }

    public static void main(String[] args) {
        int[][] obstacleGrid = { { 0, 0, 0 }, { 0, 1, 0 }, { 0, 0, 0 } };
        System.out.println(new Solution().uniquePathsWithObstacles(obstacleGrid));
    }

}

 

 

参考：

http://blog.csdn.net/kenden23/article/details/17303497

http://blog.csdn.net/kenden23/article/details/17317805