public static void main(String[] args) {
        int[][] population = {{1, 2, 3}, {2, 3, 4}, {3, 2, 1}};
        int x = 0; int y = 0, strength = 2;

        printPopulation(answer(population, x, y, strength));
    }

    private static int[][] answer(int[][] population, int x, int y, int strength) {
        checkInfected(population, x, y, strength);
        return population;
    }

    private static void checkSurroundings(int[][] population, int x, int y, int strength) {
        if(x != 0)
            checkInfected(population, x-1, y, strength);
        if(y != 0)
            checkInfected(population, x, y-1, strength);
        if(x != population[y].length)
            checkInfected(population, x+1, y, strength);
        if(y != population.length)
            checkInfected(population, x, y+1, strength);
    }

    private static void checkInfected(int[][] population, int x, int y, int strength) {
        if(population[y][x] <= strength) {
            population[y][x] = -1;
            checkSurroundings(population, x, y, strength);
        }
    }

    private static void printPopulation(int[][] population) {
        for (int[] row : population) {
            for (int val : row) {
                System.out.print("|" + val + "|");
            }
            System.out.println();
        }

    }