private static final int ZOMBIE = 1;
private static final int[][] DIRS = {{0, 1}, {1, 0}, {0, -1}, {-1, 0}};

public int minHours(List<List<Integer>> grid) {
    int people = 0;
    Queue<Point> zombies = new ArrayDeque<>();
    for (int r = 0; r < grid.size(); r++) {
        for (int c = 0; c < grid.get(0).size(); c++) {
            if (grid.get(r).get(c) == ZOMBIE) {
                zombies.add(new Point(r, c));
            } else {
                people++;
            }
        }
    }

    if (people == 0) return 0;

    for (int hours = 1; !zombies.isEmpty(); hours++) {
        for (int sz = zombies.size(); sz > 0; sz--) {
            Point zombie = zombies.poll();

            for (int[] dir : DIRS) {
                int r = zombie.r + dir[0];
                int c = zombie.c + dir[1];

                if (isHuman(grid, r, c)) {
                    people--;
                    if (people == 0) return hours;
                    grid.get(r).set(c, ZOMBIE);
                    zombies.add(new Point(r, c));
                }
            }
        }
    }
    return -1;
}

private boolean isHuman(List<List<Integer>> grid, int r, int c) {
    return r >= 0 && r < grid.size() &&c >= 0 && c < grid.get(0).size() && grid.get(r).get(c) != ZOMBIE;
}