20200319-leetcode-1162. 地圖分析(BFS)

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class Solution(object):
    def maxDistance(self, grid):
        """
        :type grid: List[List[int]]
        :rtype: int
        """
        n,m=len(grid),len(grid[0])
        cnt=0
        visited=[[0]*m for _ in range(n)]
        minDis=[[999999]*m for _ in range(n)]
        q=[]

        for i in range(n):
            for j in range(m):
                if grid[i][j]==1:
                    visited[i][j]=1
                    minDis[i][j]=0
                    q.append((i,j))
                    cnt=cnt+1
        #是否全部是海洋陸地
        if cnt==0 or cnt==m*m:
            return -1

        ans = 0
        while q:
            x,y=q.pop(0)    #取出隊首元素
            for i,j in [(x+1,y),(x-1,y),(x,y+1),(x,y-1)]:
                if 0<=i<n and 0<=j<m and visited[i][j]==0:
                    q.append((i,j))
                    visited[i][j]=1
                    minDis[i][j]=minDis[x][y]+1
                    ans=max(ans,minDis[i][j])
        
        return ans