题目:
You are given a m x n 2D grid initialized with these three possible values.
-1 - A wall or an obstacle.
0 - A gate.
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
解答:
每一次加入进来的结点,都时当前位置gate可到达的最短距离。用BFS逐层扫。
public void wallsAndGates(int[][] rooms) {
if (rooms == null || rooms.length == 0 || rooms[0].length == 0) return;
Queue<int[]> q = new LinkedList<>();
int m = rooms.length, n = rooms[0].length;
for (int i = 0; i < m; i++) {
for (int j = 0; j < n; j++) {
if (rooms[i][j] == 0) {
q.add(new int[]{i, j});
}
}
}
int[][] dir = {{-1, 0},{1, 0},{0, 1},{0, -1}};
while (!q.isEmpty()) {
int[] curt = q.poll();
int i = curt[0], j = curt[1];
for (int k = 0; k < dir.length; k++) {
int x = i + dir[k][0], y = j + dir[k][1];
if (x < 0 || x > m - 1 || y < 0 || y > n - 1 || rooms[x][y] != Integer.MAX_VALUE) {
continue;
}
rooms[x][y] = rooms[i][j] + 1;
q.add(new int[] {x, y});
}
}
}
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