01 Matrix.java

class Solution {
  public int[][] updateMatrix(int[][] mat) {
    final int m = mat.length;
    final int n = mat[0].length;
    final int[] dirs = new int[] {0, 1, 0, -1, 0};
    Queue<int[]> q = new ArrayDeque<>();
    boolean[][] seen = new boolean[m][n];

    for (int i = 0; i < m; ++i)
      for (int j = 0; j < n; ++j)
        if (mat[i][j] == 0) {
          q.offer(new int[] {i, j});
          seen[i][j] = true;
        }

    while (!q.isEmpty()) {
      final int i = q.peek()[0];
      final int j = q.poll()[1];
      for (int k = 0; k < 4; ++k) {
        final int x = i + dirs[k];
        final int y = j + dirs[k + 1];
        if (x < 0 || x == m || y < 0 || y == n)
          continue;
        if (seen[x][y])
          continue;
        mat[x][y] = mat[i][j] + 1;
        q.offer(new int[] {x, y});
        seen[x][y] = true;
      }
    }

    return mat;
  }
}