public class Solution {
    public int[][] updateMatrix(int[][] matrix) {
        if (matrix == null || matrix.length == 0) {
            return null;
        }
        int[][] diff = {{1, 0}, {-1, 0}, {0, 1}, {0, -1}};
        Queue<Integer> queue = new LinkedList<>();
        int[][] result = new int[matrix.length][matrix[0].length];
        for (int i = 0; i < result.length; i++) {
            for (int j = 0; j < result[0].length; j++) {
                if (matrix[i][j] == 0) {
                    result[i][j] = 0;
                    queue.add(i * result[0].length + j);
                } else {
                    result[i][j] = Integer.MAX_VALUE;
                }
            }
        }
        while (!queue.isEmpty()) {
            int index = queue.poll();
            int i = index / result[0].length;
            int j = index - i * result[0].length;
            for (int[] current : diff) {
                int x = i + current[0];
                int y = j + current[1];
                if (x < 0 || x >= result.length || y < 0 || y >= result[0].length || result[x][y] <= result[i][j]) {
                    continue;
                }
                result[x][y] = result[i][j] + 1;
                queue.offer(x * result[0].length + y);
            }
        }
        return result;
    }
}