边工作边刷题：70天一遍leetcode: day 38

number of islands

错误点

• 没检查grid element是否为1，光想着边界检测和visited check了，总共是3个条件
• visited check时机？最好是在dfs外面，在dfs里面设为true
• 小错误：grid的值是字符

class Solution(object):
    def numIslands(self, grid):
        """
        :type grid: List[List[str]]
        :rtype: int
        """
        def dfs(grid, visited, x, y):
            m = len(grid)
            n = len(grid[0])
            visited[x*n+y]=True
            dir = [[1,0],[0,1],[-1,0],[0,-1]]
            for d in dir:
                x1=x+d[0]
                y1=y+d[1]
                if x1<m and x1>=0 and y1<n and y1>=0 and 
                   not visited[x1*n+y1] and grid[x1][y1]=='1':
                   dfs(grid, visited, x1, y1)
        
        if not grid: return 0
        m = len(grid)
        n = len(grid[0])
        count = 0
        visited = [False]*(m*n)
        for x in range(m):
            for y in range(n):
                if not visited[x*n+y] and grid[x][y]=='1':
                    count+=1
                    dfs(grid, visited, x, y)
        
        return count