leetcode 463 python

You are given a map in form of a two-dimensional integer grid where 1 represents land and 0 represents water. Grid cells are connected horizontally/vertically (not diagonally). The grid is completely surrounded by water, and there is exactly one island (i.e., one or more connected land cells). The island doesn't have "lakes" (water inside that isn't connected to the water around the island). One cell is a square with side length 1. The grid is rectangular, width and height don't exceed 100. Determine the perimeter of the island.

[[0,1,0,0],
[1,1,1,0],
[0,1,0,0],
[1,1,0,0]]

Answer: 16

class Solution(object):    def islandPerimeter(self, grid):        """        :type grid: List[List[int]]        :rtype: int        """        a = len(grid)        b = len(grid[0])              t = 0        for x in range(a):                      for z in range(b):               #print("%d",j)                                if grid[x][z]==0:                    continue                t = t + 4                if x>0 and grid[x-1][z]==1:                    t = t -2                if z>0 and grid[x][z-1]==1:                    t = t -2               return t

def islandPerimeter(self, grid):
    return sum(sum(map(operator.ne, [0] + row, row + [0]))
               for row in grid + map(list, zip(*grid)))

Since there are no lakes, every pair of neighbour cells with different values is part of the perimeter (more precisely, the edge between them is). So just count the differing pairs, both horizontally and vertically (for the latter I simply transpose the grid).

我们先看map。map()函数接收两个参数，一个是函数，一个是Iterable，map将传入的函数依次作用到序列的每个元素，并把结果作为新的Iterator返回。

举例说明，比如我们有一个函数f(x)=x²，要把这个函数作用在一个list[1, 2, 3, 4, 5, 6, 7, 8, 9]上，就可以用map()实现如下：

现在，我们用Python代码实现：

>>> def f(x):...     return x * x...>>> r = map(f, [1, 2, 3, 4, 5, 6, 7, 8, 9])>>> list(r)[1, 4, 9, 16, 25, 36, 49, 64, 81]

sum()的参数是一个list

python中的operator库 

ne(...)         ne(a, b) -- Same as  a!=b.

zip()是Python的一个内建函数，它接受一系列可迭代的对象作为参数，将对象中对应的元素打包成一个个tuple（元组），然后返回由这些tuples组成的list（列表）。若传入参数的长度不等，则返回list的长度和参数中长度最短的对象相同。利用*号操作符，可以将list unzip（解压），看下面的例子就明白了：

>>> a = [1,2,3]
>>> b = [4,5,6]
>>> c = [4,5,6,7,8]
>>> zipped = zip(a,b)
[(1, 4), (2, 5), (3, 6)]
>>> zip(a,c)
[(1, 4), (2, 5), (3, 6)]
>>> zip(*zipped)
[(1, 2, 3), (4, 5, 6)]