Zombie Matrix

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; } private static class Point { int r, c; Point(int r, int c) { this.r = r; this.c = c; } }