Leetcode #286. Walls and Gates

[yokostan]

DFS:

class Solution {
    public int[] pos = {0, 1, 0, -1, 0};

    public void wallsAndGates(int[][] rooms) {
        for (int i = 0; i < rooms.length; i++) {
            for (int j = 0; j < rooms[0].length; j++) {
                if (rooms[i][j] == 0) {
                    dfs(rooms, i, j);
                } 
            }
        }
    }

    private void dfs(int[][] rooms, int i, int j) {
        for (int k = 0; k < 4; k++) {
            int p = i + pos[k], q = j + pos[k + 1];
            if (0 <= p && p < rooms.length && 0 <= q && q < rooms[0].length && rooms[p][q] > rooms[i][j] + 1) {
                rooms[p][q] = rooms[i][j] + 1;
                dfs(rooms, p, q);
            }
        }
    }
}

This position index array initialization is interesting. Also, how we judge the minimum distance is important rooms[p][q] > rooms[i][j] + 1. Here we start from the gates!

BFS (slower):

class Solution {
    public int[] pos = {0, 1, 0, -1, 0};

    public void wallsAndGates(int[][] rooms) {
        if (rooms.length == 0 || rooms[0].length == 0) return ;
        int m = rooms.length, n = rooms[0].length;
        Queue<int[]> queue = new LinkedList<>();

        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                if (rooms[i][j] == 0) queue.add(new int[]{i, j});
            }
        }

        while (!queue.isEmpty()) {
            int[] top = queue.remove();
            int row = top[0], col = top[1];
            if (row > 0 && rooms[row - 1][col] == Integer.MAX_VALUE) {
                rooms[row - 1][col] = rooms[row][col] + 1;
                queue.add(new int[]{row - 1, col});
            }
            if (row < rooms.length - 1 && rooms[row + 1][col] == Integer.MAX_VALUE) {
                rooms[row + 1][col] = rooms[row][col] + 1;
                queue.add(new int[]{row + 1, col});
            }
            if (col > 0 && rooms[row][col - 1] == Integer.MAX_VALUE) {
                rooms[row][col - 1] = rooms[row][col] + 1;
                queue.add(new int[]{row, col - 1});
            }
            if (col < rooms[0].length - 1 && rooms[row][col + 1] == Integer.MAX_VALUE) {
                rooms[row][col + 1] = rooms[row][col] + 1;
                queue.add(new int[]{row, col + 1});
            }
        }
    }
}