LeetCode 1020 Number of Enclaves

You are given an m x n binary matrix grid, where 0 represents a sea cell and 1 represents a land cell.

A move consists of walking from one land cell to another adjacent (4-directionally) land cell or walking off the boundary of the grid.

Return the number of land cells in grid for which we cannot walk off the boundary of the grid in any number of moves.

Example 1:

Input: grid = [[0,0,0,0],[1,0,1,0],[0,1,1,0],[0,0,0,0]]

Output: 3

Explanation: There are three 1s that are enclosed by 0s, and one 1 that is not enclosed because its on the boundary.

Example 2:

Input: grid = [[0,1,1,0],[0,0,1,0],[0,0,1,0],[0,0,0,0]]

Output: 0

Explanation: All 1s are either on the boundary or can reach the boundary.

Constraints:

• m == grid.length

• n == grid[i].length

• 1 <= m, n <= 500

• grid[i][j] is either 0 or 1.

Accepted

95,195

Submissions

145,440

DFS 一定要有返回的条件，不然直接就内存溢出了。。。

相当于把边上为1的数字直接改成0，当然包含跟他相邻的数字，

然后再计算剩下1的数量，返回即可。

Runtime: 9 ms, faster than 76.21% of Java online submissions for Number of Enclaves.

Memory Usage: 53.8 MB, less than 76.65% of Java online submissions for Number of Enclaves.