1162. As Far from Land as Possible

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https://leetcode.com/problems/as-far-from-land-as-possible/

Given an N x N grid containing only values 0 and 1, where 0 represents water and 1 represents land, find a water cell such that its distance to the nearest land cell is maximized and return the distance.

The distance used in this problem is the Manhattan distance: the distance between two cells (x0, y0) and (x1, y1) is |x0 - x1| + |y0 - y1|.

If no land or water exists in the grid, return -1.

 

Example 1:

Input: [[1,0,1],[0,0,0],[1,0,1]]
Output: 2
Explanation: 
The cell (1, 1) is as far as possible from all the land with distance 2.

Example 2:

Input: [[1,0,0],[0,0,0],[0,0,0]]
Output: 4
Explanation: 
The cell (2, 2) is as far as possible from all the land with distance 4.

 

Note:

  1. 1 <= grid.length == grid[0].length <= 100
  2. grid[i][j] is 0 or 1
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Intuition
Shortest path => BFS, when exiting at first match
Longest path => BFS, so that we can count levels, and exit till q is Empty

Instead of starting from water 0, start with 1 land, and count levels
Because last increment to level is extra.. q becomes empty, return ans - 1 to avoid off by 1

Modify the input grid to avoid extra space

Corner case - check the initial q size 0 or M * N to return -1
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Time - O(M * N)
Space - O(1)
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