A rectangular field is a grid field: 1 is grass a sheep may stand on and 0 is a wall. A sheep walks
between side-adjacent grass cells and can step off the field from any grass cell on the outer border.
Return the number of grass cells from which a sheep can never leave the field.
Examples
Input: field = [[0,0,0,0],
[1,0,1,0],
[0,1,1,0],
[0,0,0,0]]
Output: 3
Explanation: (1,0) is on the border, so a sheep there walks off. The three grass cells (1,2), (2,1), (2,2)
form a pocket with no border cell.
Input: field = [[1,1],[1,1]]
Output: 0
Constraints
1 <= rows, cols <= 60- Target
O(rows * cols)time.
Goals
- Search backwards from all exits instead of forwards from every cell
- Count cells never reached by a multi-source search