python leetcode 200. Number of Islands

记忆化搜索+DFS 此类问题的关键是找准遍历方式

class Solution(object):
    def numIslands(self, grid):
        """
        :type grid: List[List[str]]
        :rtype: int
        """
        m = len(grid)
        if m==0:
            return 0 
        n = len(grid[0])
        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(x,y):
            nbrow = [1,0,-1,0]
            nbcol = [0,1,0,-1]
            for k in range(4):
                newx= x + nbrow[k]
                newy= y + nbcol[k]
                if check(newx,newy):
                    visit[newx][newy]=True
                    dfs(newx,newy)
        count = 0 
        for row in range(m):
            for col in range(n):
                if check(row,col):
                    visit[row][col] = True
                    dfs(row,col)
                    count +=1 
        return count