Leetcode 1162. As Far from Land as Possible

题目描述：

Given an N x N grid containing only values 0 and 1, where 0 represents water and 1represents 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

 题解：

很显然这个题使用BFS，但是如果遍历矩阵每次遇到0开始一次BFS时间复杂度太差，达到O(mn*mn)。解决办法时一次将所有的0的坐标放入队列，这样能将复杂度降低到O(mn)。

代码如下：

class Solution {
    public int maxDistance(int[][] grid) {
        int m = grid.length, n = grid[0].length;
        boolean[][] visited = new boolean[m][n];
        Queue<int[]> q = new LinkedList<>();
        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                if (grid[i][j] == 1) {
                    visited[i][j] = true;
                    q.offer(new int[]{i, j});
                }
            }
        }
        int[][] dirs = new int[][]{{1, 0}, {0, 1}, {-1, 0}, {0, -1}};
        int result = -1;
        while (!q.isEmpty()) {
            int size = q.size();
            while (size-- > 0) {
                int[] cur = q.poll();
                result = Math.max(result, grid[cur[0]][cur[1]] - 1);
                for (int[] dir : dirs) {
                    int x = cur[0] + dir[0], y = cur[1] + dir[1];
                    if (x >= 0 && x < m && y >= 0 && y < n && !visited[x][y]) {
                        visited[x][y] = true;
                        grid[x][y] = grid[cur[0]][cur[1]] + 1;
                        q.offer(new int[]{x, y});
                    }
                }
            }
        }
        return result == 0 ? -1 : result;
    }
}