286. Walls and Gates


You are given a m x n 2D grid initialized with these three possible values.

  1. -1 - A wall or an obstacle.
  2. 0 - A gate.
  3. INF - Infinity means an empty room. We use the value 231 - 1 = 2147483647 to represent INF as you may assume that the distance to a gate is less than 2147483647.

Fill each empty room with the distance to its nearest gate. If it is impossible to reach a gate, it should be filled with INF.

For example, given the 2D grid:

INF  -1  0  INF
INF INF INF  -1
INF  -1 INF  -1
  0  -1 INF INF

After running your function, the 2D grid should be:

  3  -1   0   1
  2   2   1  -1
  1  -1   2  -1
  0  -1   3   4


Solution

Approach #1 (Brute Force) [Time Limit Exceeded]

The brute force approach is simple, we just implement a breadth-first search from each empty room to its nearest gate.

While we are doing the search, we use a 2D array called distance to keep track of the distance from the starting point. It also implicitly tell us whether a position had been visited so it won't be inserted into the queue again.

private static final int EMPTY = Integer.MAX_VALUE;
private static final int GATE = 0;
private static final int WALL = -1;
private static final List<int[]> DIRECTIONS = Arrays.asList(
        new int[] { 1,  0},
        new int[] {-1,  0},
        new int[] { 0,  1},
        new int[] { 0, -1}
);

public void wallsAndGates(int[][] rooms) {
    if (rooms.length == 0) return;
    for (int row = 0; row < rooms.length; row++) {
        for (int col = 0; col < rooms[0].length; col++) {
            if (rooms[row][col] == EMPTY) {
                rooms[row][col] = distanceToNearestGate(rooms, row, col);
            }
        }
    }
}

private int distanceToNearestGate(int[][] rooms, int startRow, int startCol) {
    int m = rooms.length;
    int n = rooms[0].length;
    int[][] distance = new int[m][n];
    Queue<int[]> q = new LinkedList<>();
    q.add(new int[] { startRow, startCol });
    while (!q.isEmpty()) {
        int[] point = q.poll();
        int row = point[0];
        int col = point[1];
        for (int[] direction : DIRECTIONS) {
            int r = row + direction[0];
            int c = col + direction[1];
            if (r < 0 || c < 0 || r >= m || c >= n || rooms[r][c] == WALL
                    || distance[r][c] != 0) {
                continue;
            }
            distance[r][c] = distance[row][col] + 1;
            if (rooms[r][c] == GATE) {
                return distance[r][c];
            }
            q.add(new int[] { r, c });
        }
    }
    return Integer.MAX_VALUE;
}

Complexity analysis

  • Time complexity : . For each point in the size grid, the gate could be at most steps away.

  • Space complexity : . The space complexity depends on the queue's size. Since we won't insert points that have been visited before into the queue, we insert at most points into the queue.

Approach #2 (Breadth-first Search) [Accepted]

Instead of searching from an empty room to the gates, how about searching the other way round? In other words, we initiate breadth-first search (BFS) from all gates at the same time. Since BFS guarantees that we search all rooms of distance d before searching rooms of distance d + 1, the distance to an empty room must be the shortest.

private static final int EMPTY = Integer.MAX_VALUE;
private static final int GATE = 0;
private static final List<int[]> DIRECTIONS = Arrays.asList(
        new int[] { 1,  0},
        new int[] {-1,  0},
        new int[] { 0,  1},
        new int[] { 0, -1}
);

public void wallsAndGates(int[][] rooms) {
    int m = rooms.length;
    if (m == 0) return;
    int n = rooms[0].length;
    Queue<int[]> q = new LinkedList<>();
    for (int row = 0; row < m; row++) {
        for (int col = 0; col < n; col++) {
            if (rooms[row][col] == GATE) {
                q.add(new int[] { row, col });
            }
        }
    }
    while (!q.isEmpty()) {
        int[] point = q.poll();
        int row = point[0];
        int col = point[1];
        for (int[] direction : DIRECTIONS) {
            int r = row + direction[0];
            int c = col + direction[1];
            if (r < 0 || c < 0 || r >= m || c >= n || rooms[r][c] != EMPTY) {
                continue;
            }
            rooms[r][c] = rooms[row][col] + 1;
            q.add(new int[] { r, c });
        }
    }
}

Complexity analysis

  • Time complexity : .

    If you are having difficulty to derive the time complexity, start simple.

    Let us start with the case with only one gate. The breadth-first search takes at most steps to reach all rooms, therefore the time complexity is . But what if you are doing breadth-first search from gates?

    Once we set a room's distance, we are basically marking it as visited, which means each room is visited at most once. Therefore, the time complexity does not depend on the number of gates and is .

  • Space complexity : . The space complexity depends on the queue's size. We insert at most points into the queue.

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