function getPaths (obstacleGrid) {
  let m = obstacleGrid.length
  let n = obstacleGrid[0].length

  // 极端条件
  if (obstacleGrid[0][0] === 1 || obstacleGrid[m - 1][n - 1] === 1) {
    return 0
  }

  obstacleGrid[0][0] = 1

  // 将第一行遍历，设置为0代表不能通过，设置为1代表可以通过
  for (let i = 1; i < n; i++) {
    // 当前格为0且前一个格为1，代表可以通过；当前格为1 || 前一个格为0，代表不能通过
    obstacleGrid[0][i] = obstacleGrid[0][i] === 0 && obstacleGrid[0][i - 1] === 1 ? 1 : 0
  }

  // 同理遍历第一列
  for (let j = 1; j < m; j++) {
    obstacleGrid[j][0] = obstacleGrid[j][0] === 0 && obstacleGrid[j - 1][0] === 1 ? 1 : 0
  }

  // 从上向下，从左往右遍历，f(m,n) = f(m-1,n) + f(m,n-1)
  for (let i = 1; i < m; i++) {
    for (let j = 1; j < n; j++) {
      obstacleGrid[i][j] = obstacleGrid[i][j] !== 1 ? obstacleGrid[i - 1][j] + obstacleGrid[i][j - 1] : 0
    }
  }

  return obstacleGrid[m - 1][n - 1]
}

参考地址：https://leetcode.com/problems/unique-paths-ii/solution/