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 as1and0respectively 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 is2.

分析

当前点可达时，两个方向的path相加，不可达则reset为0。

如果初始状态可达，则f[1][1]是1，要单独判断。

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