[Leetcode]@python 63. Unique Paths II

题目链接：https://leetcode.com/problems/unique-paths-ii/

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 题目大意：给定mxn的矩阵，0表示可以通过，1表示不可以通过，返回从（0，0）到（m，n）一共有多少种走法

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 解题思路：考虑使用DP，给定一个矩阵a[i][j]表示从（0,0）到（i,j）共有多少种走法，动态转移方程：a[i][j] = a[i-1][j]+a[i][j-1]，需注意考虑在矩阵边界的是情况

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  代码（python）：

class Solution(object):
    def uniquePathsWithObstacles(self, obstacleGrid):
        """
        :type obstacleGrid: List[List[int]]
        :rtype: int
        """
        m = len(obstacleGrid)
        n = len(obstacleGrid[0])
        ans = [[0 for i in range(n)] for j in range(m)]
        ans[0][0] = 1
        for i in range(m):
            for j in range(n):
                if obstacleGrid[i][j] == 1:
                    ans[i][j] = 0
                elif i != 0 and j == 0:
                    ans[i][j] = ans[i - 1][j]
                elif i == 0 and j != 0:
                    ans[i][j] = ans[i][j - 1]
                elif i != 0 and j != 0:
                    ans[i][j] = ans[i - 1][j] + ans[i][j - 1]
        return ans[m - 1][n - 1]