leetcode1091

Posted 2022-01-14 asenyang

这道题使用dfs会超时，看评论区也有人遇到同样的问题，比赛时调试了1个多小时尝试改进，没有意识到应该换用非递归的bfs可以解决，消耗了大量的时间。

超时的方案如下，使用python实现：（经过尝试，能通过的测试用例中，使用5个方向就可以了）

 1 import sys
 2 class Solution:
 3     def __init__(self):
 4         self.visited = []
 5         self.distance = sys.maxsize
 6         #self.direction = [[-1,-1],[-1,0],[-1,1],[0,-1],[0,1],[1,-1],[1,0],[1,1]]
 7         self.direction = [[1,1],[0,1],[1,0],[-1,1],[1,-1]]
 8         #self.direction = [[1,1],[0,1],[1,0],[1,-1]]
 9         #self.direction = [[1,1],[0,1],[1,0]]
10 
11     def dfs(self,grid,N,i,j,distance):
12         if i == N-1 and j == N-1:
13             self.distance = min(self.distance,distance+1)
14             return
15 
16         if distance >= self.distance:
17             return
18         
19         if self.visited[i][j] == 0:
20             self.visited[i][j] = 1
21             distance += 1
22             for direct in self.direction:
23                 x = i + direct[0]
24                 y = j + direct[1]
25                 if x < 0 or x >= N or y < 0 or y >= N:
26                     continue
27                 if grid[x][y] == 1 or self.visited[x][y] == 1:
28                     continue
29                 self.dfs(grid,N,x,y,distance)
30             distance -= 1
31             self.visited[i][j] = 0
32 
33 
34     def shortestPathBinaryMatrix(self, grid: ‘List[List[int]]‘) -> int:
35         N = len(grid)
36         if grid[0][0] == 1 or grid[N-1][N-1] == 1:
37             return -1
38         self.visited = [[0 for _ in range(N)]for _ in range(N)]
39         
40         self.dfs(grid,N,0,0,0)
41         return self.distance if self.distance < sys.maxsize else -1

暂时先这样吧。