Number of Islands

Given a 2d grid map of '1's (land) and '0's (water), count the number of islands. An island is surrounded by water and is formed by connecting adjacent lands horizontally or vertically. You may assume all four edges of the grid are all surrounded by water.

Example 1:

11110
11010
11000
00000

Answer: 1

Example 2:

11000
11000
00100
00011

Answer: 3

DFS 遍历，将1周围的1都变为0，最后数下有多少个1即可。

class Solution(object):
    def numIslands(self, grid):
        """
        :type grid: List[List[str]]
        :rtype: int
        """
        if not len(grid):
            return 0
        res = 0
        m,n = len(grid),len(grid[0])
        for i in range(m):
            for j in range(n):
                if grid[i][j]=='':
                    res += 1
                    self.dfs(grid,i,j,m,n)
        return res

    def dfs(self,grid,i,j,m,n):
        if i<0 or i>=m or j<0 or j>=n: return
        if grid[i][j]=='':
            grid[i][j]=''
            dz = zip([1,0,-1,0],[0,1,0,-1])
            for px,py in dz:
                self.dfs(grid,i+px,j+py,m,n)