[LintCode] 598. Zombie in Matrix

Given a 2D grid, each cell is either a wall 2, a zombie 1 or people 0 (the number zero, one, two).Zombies can turn the nearest people(up/down/left/right) into zombies every day, but can not through wall. How long will it take to turn all people into zombies? Return -1 if can not turn all people into zombies.

Example

Example 1:

Input:
[[0,1,2,0,0],
 [1,0,0,2,1],
 [0,1,0,0,0]]
Output:
2

Example 2:

Input:
[[0,0,0],
 [0,0,0],
 [0,0,1]]
Output:
4

public class Solution {
    /**
     * @param grid: a 2D integer grid
     * @return: an integer
     */
    int row;
    int col;
    public int zombie(int[][] grid) {
        // write your code here
        
        row = grid.length;
        col = grid[0].length;
        int numPeople = 0;
        Queue<Cell> queue = new LinkedList<>();
    
        for (int i = 0; i < row; i++) {
            for (int j = 0; j < col; j++) {
                if (grid[i][j] == 1) {
                    queue.add(new Cell(i, j, grid[i][j]));
                    
                } else if (grid[i][j] == 0) {
                    numPeople += 1;
                }
            }
        }
        int[][] directions = new int[][]{{-1, 0}, {1, 0}, {0, -1}, {0, 1}};
        int res = 0;
        while(!queue.isEmpty()) {
            int size = queue.size();
            if (numPeople == 0) {
                return res;
            }
            // for (int i = 0; i < row; i++) {
            //     System.out.println(Arrays.toString(grid[i]));
            // }
            // System.out.println();
            while (size-- > 0) {
                Cell cur = queue.poll();
                for (int[] direction: directions) {
                    int nxtX = cur.x + direction[0];
                    int nxtY = cur.y + direction[1];
                    if (isValid(nxtX, nxtY, grid)) {
                        Cell nxtCell = new Cell(nxtX, nxtY, grid[nxtX][nxtY]);
                        queue.offer(nxtCell);
                        grid[nxtX][nxtY] = 1;
                        numPeople -= 1;
                    }
                }
            }
            res += 1;
        }
        return -1;
    }
    
    private boolean isValid(int x, int y, int[][] grid) {
        if (x >= 0 && x < row && y >= 0 && y < col && grid[x][y] == 0) {
            return true;
        }
        return false;
    }
}

class Cell {
    int x;
    int y;
    int val;
    public Cell(int x, int y, int val) {
        this.x = x;
        this.y = y;
        this.val = val;
    }
}