200. Number of Islands

class Solution(object):
    def numIslands(self, grid):
        """
        :type grid: List[List[str]]
        :rtype: int
        """
        if not grid: return 0
        m=len(grid)
        n=len(grid[0])
    
        directions=[(0,1),(1,0),(0,-1),(-1,0)]
        def dfs(i,j):
            grid[i][j]='0'
            for direction in directions:
                x,y=i+direction[0],j+direction[1]
                if x<0 or x>m-1 or y<0 or y>n-1 or grid[x][y]=='0':
                    continue 
                dfs(x,y)
            
        count=0
        for i in xrange(m):
            for j in xrange(n):
                if grid[i][j]=='1':
                    count+=1
                    dfs(i,j)
        return count