LeetCode 1091. Shortest Path in Binary Matrix
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原题链接在这里:https://leetcode.com/problems/shortest-path-in-binary-matrix/
题目:
In an N by N square grid, each cell is either empty (0) or blocked (1).
A clear path from top-left to bottom-right has length k
if and only if it is composed of cells C_1, C_2, ..., C_k
such that:
- Adjacent cells
C_i
andC_{i+1}
are connected 8-directionally (ie., they are different and share an edge or corner) C_1
is at location(0, 0)
(ie. has valuegrid[0][0]
)C_k
is at location(N-1, N-1)
(ie. has valuegrid[N-1][N-1]
)- If
C_i
is located at(r, c)
, thengrid[r][c]
is empty (ie.grid[r][c] == 0
).
Return the length of the shortest such clear path from top-left to bottom-right. If such a path does not exist, return -1.
Example 1:
Input: [[0,1],[1,0]]
Output: 2
Example 2:
Input: [[0,0,0],[1,1,0],[1,1,0]]
Output: 4
Note:
1 <= grid.length == grid[0].length <= 100
grid[r][c]
is0
or1
题解:
When doing BFS, for each level, mark the current size.
When polling current size node, pop up to next level.
If current polled node is bottom right node, return level.
Otherwise, for its 8 surrounding node, check if it is not visited nor block, add it to the queue.
Time Complexity: O(m*n). m = grid.length. n = grid[0].length.
Space: O(m*n).
AC Java:
1 class Solution { 2 public int shortestPathBinaryMatrix(int[][] grid) { 3 if(grid == null || grid.length == 0 || grid[0].length == 0){ 4 return -1; 5 } 6 7 int m = grid.length; 8 int n = grid[0].length; 9 if(grid[0][0] == 1 || grid[m-1][n-1] == 1){ 10 return -1; 11 } 12 13 LinkedList<int []> que = new LinkedList<>(); 14 boolean [][] visited = new boolean[m][n]; 15 visited[0][0] = true; 16 que.add(new int[]{0, 0}); 17 18 int level = 1; 19 while(!que.isEmpty()){ 20 int size = que.size(); 21 while(size-->0){ 22 int [] cur = que.poll(); 23 if(cur[0] == m-1 && cur[1] == n-1){ 24 return level; 25 } 26 27 for(int i = -1; i<=1; i++){ 28 for(int j = -1; j<=1; j++){ 29 int x = cur[0]+i; 30 int y = cur[1]+j; 31 if(x<0 || x>=m || y<0 || y>=n || visited[x][y] || grid[x][y]==1){ 32 continue; 33 } 34 35 visited[x][y] = true; 36 que.add(new int[]{x, y}); 37 } 38 } 39 } 40 41 level++; 42 } 43 44 return -1; 45 } 46 }
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