python 学习 leetcode ---number of island

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

思路 = dfs（四叉树，上下左右） +剪枝

剪枝是剪掉遍历过的结点

class Solution(object):
    def numIslands(self, grid):
        """
        :type grid: List[List[str]]
        :rtype: int
        """
        count=0
        m=len(grid)
        if m==0:
            return  0
        n=len(grid[0])
        #visit=[[False]*n]*m
        visit = [[False for i in range(n)]for j in range(m)]
        def check(x, y):
            if x >= 0 and x<m and y>= 0 and y< n and grid[x][y] == '1' and visit[x][y] == False:
                return True
        def dfs(i ,j):
            nbrow = [1,0,-1,0]
            nbcol = [0,1,0,-1]
            for d in range(4):
                ni=i+nbrow[d]
                nj=j+nbcol[d]
                if(check(ni,nj)==True):
                    visit[ni][nj]=True
                    dfs(ni,nj)
        for i in range(m):
            for j in range(n):
                if(check(i,j)==True):
                    visit[i][j]=True
                    dfs(i,j)
                    count+=1
        return count